This may be kinda easy, but i need help to solve a function with p(x)=ax+b, p(-1)=3, i have tried but can't seem to fix it.
Solve[{ax+b}, x=-1 {a,b},x]
I don't quite understand you, but here is something that may be relevant:
p[x_] := a x + b; Solve[ p[-1] == 3]
