I am taking a EEP course and in our recent chapter we are learning about total resistance of the resistors and summation of voltage (E-V1 -V2-V3=0 etc.) or ƩV=0. Rτ = R1 + R2 + R3 +... etc. Anyhow to cut the chitchat....
problem is "The circuit of Fig. 5-7 has a current of 2.5mA (milli-amps). Find the following quantities...
"Given" R1 = 3.3kΩ R2 = ? R3 = 5.6kΩ I = 2.5(mA) E = 45V
My teacher has taught us how to do Rτ and ƩV. But he has yet to show us how to find a resistor when one is missing... either that or its so obvious that i am not grasping it.... Help please? All i need to do is find R2 and the rest i can solve on my own.

A) The total resistance of the curcuit.
B) The value of the unknown resistance (R2)

C and D i am choosing not to put on here because i know how to do them.

 

Well...look at what you know. So this is a series circuit yes?

So the voltage drops at each resistor sum up to = 45V right? Since V1 - V2 - V3.....=0

What you can do is find each voltage at the resistor, except the missing one which is R2....so 45V = V1 + V2 + V3..... and solve for V2...from there you can find R2 right. You can use the current of 2.5mA for all the resistors since they are in series.