.I just trying to understand the concept of voltage drop across a resistor. what will be the voltage at point A and B in the figure I've attached below.
Voltage is
never measured AT a point. It is
always measured BETWEEN two points. The voltage drop across a resistor is given by Ohm's Law to be the product of the current
through the resistor and the resistance value:
V = (I)(R), where V is measured in volts, I is measured in amperes, and R is measured in ohms.
You have labeled "Votage at this point = 6V" but, as others have stated, this is meaningless. Then you later said this is measured with respect to "ground" but do not show on the schematic where the "ground" is. As it stands, without knowing what is on the remainder of the schematic, where you have written "This point goes to the ADC pin of a mcu" there is no way to determine what current (if any) is present in the resistors. If there is a current present, it will be the same current in both resistors because they are connected in series. This is a consequence of Kirchoff's Current Law: Whatever currents enter a node must also leave that node, paying close attention to algebraic signs. Or stated another way, the algebraic sum of all the currents entering or leaving a node must be zero. A node is any point on a schematic diagram to which two or more wires are attached. The red points on your diagram, two of which are labeled "A" and "B" are nodes.
With respect to "ground" the ADC input represents an additional resistance in series with the two you have drawn and that resistance, along with the "6V" input (with respect to "ground") will determine the current through both resistors and the ADC input. That current will be 6V divided by the sum of all three resistances, the two you have shown plus the ADC input resistance. Some ADCs on MCUs have relatively low input resistances, on the order of a few thousand ohms, which can affect the accuracy with which you can measure the "6V" input because the current drawn by these low input-resistance ADCs causes a voltage drop across the other resistances in series with the ADC input. Read the datasheet for the MCU to find out what the ADC input resistance is and then calculate what effect that has on your input voltage measurement.