Question 905846
You have two equations and two unknowns. 
It should be solvable.
Use one equation and substitute into the other equation.
{{{C=a+b(C+I+G)}}}
{{{C=a+bC+bI+bG}}}
{{{C-bC=a+bI+bG}}}
{{{C(1-b)=a+b(I+G)}}}
{{{C=(a+b(I+G))/(1-b)}}}
Then,
{{{Y=(a+b(I+G))/(1-b)+I+G}}}