Use Ohm's Law
Rt=V/I
Rt=10V/.018(18mA)=256ohms (total resistance)
So we get the total resistance Rt
Since there is another resistor in the circuit
R1=470ohms, R2=?
R2=Rt-R1
R2=556-470= 86ohms, But I don't know how to figure it for a parallel circuit with 3 resistors to get the setting for R3. I know what the answer is, but I don't know how to come up with it. It is 42ohms. I found it by putting numbers in until I got down to 36mA total resistance or by rounding it. Iused a TI-83 Plus calculator and by using the x-1 key for the reciprocal: an example is 2 EE 3 x-1 + 3 EE 3 x-1 = 833.333E-6 or micro ohms then you hit the x-1 key and then = 1.2kohms. EE 3 is kilo ohms EE -3 is milli.