Solution of a system with a parameter in the solution

[mazzeodalessio]

Hello guys. I have a big problem, which I can not solve. I bought a HP calculator, namely the HP Prime.
I took it especially because I have to solve several systems during a complex electronics exam, doing it all by hand would be crazy unfortunately.

I expose you the problem, I have in my system 4 equations:

Vx = Vi - I1*3
Vx = Vg + I2*5
I1 = I2 + I3
I3 = (Vx - 2Vg)/10

Now, I should calculate a output Vu = 5 - 250 *I3.

Just because I want the Vu result as a function of Vi.

By doing the hand calculation I come (right) that:

Vu = 30.66-13.16 * Vi

The problem is that I do not know how to set this calculation on the calculator, I tried in all ways using the "Solve" function but nothing. Can you give me a hand? You would solve a big problem!