This is what i have:

I tried using linearity theorem (i believe that's the correct name of the theorem):

I=E1*a+b

-80=10a+b first case

40=38a+b second case

from this equations i can find a and b

80=-10a-b

40=38a+b

120=28a a=4.29

now into first equation

-80=42.9+b b=-122.9

now i have a and b for the third case:

I=E1*a+b

I=17*4.29-122.9

I=-49.97mA

Is this correct, and is this good approach?