Help with algebra equations

Hi, I'm having trouble doing two equations. I have no idea where to start on (or how to go about doing) the 1st one, and the 2nd - well I've endlessly tried many different ways, however it always ends up coming out wrong!!

The first is:

The power developed in an electrical circuit is given by P=10I - 8I^2, where I is the current in Amps. Determine the current necessary to produce a power of 2.5 watts in the circuit.

The second is:

Applying Kirchoff's laws to an electrical circuit produces the following equations:

5=0.2I1+2(I1-I2)

12=3I2+0.4I2-(I1-I2)

Any help would be really appreciated

Thanks