Does the formula Y = C + I + G + NX ring a bell?
You have C = 150 + 0.5*Y. Substitute the function into the top equation and solve for Y.
Suppose the consumption function is C=150+0.5*Y, where C is consumption and Y is real GDP (both are in billions of $). Assuming that I=$100 billion, G=$150 billion, and NX=-$50 billion, how much is equilibrium real GDP in this economy?