GDP:Y = C + I;
Cons:C = C0 + c*Y;

sol:solve([GDP, Cons], [C, Y]);

f(C0, c, I):= ev(sol);

f(10,0.8,20);

g(x):= f(10,x,20)[1][2];

g(0.3);

rhs(x=2);
lhs(x=2);

gg(x):=rhs(f(10,x,20)[1][2]);

gg(0.3);

wxplot2d([gg(x)], [x,0.3,0.9])$