You would substitute 0.8 in for y and to get the equation 4x+8(0.8)=40. Then, you would simplify and get 4x+6.4=40. Next you would isolate x by subtracting 6.4 from both sides and get the equation 4x=33.6 and then you would divide both sides by 4 to get x=8.4.
The answer is : x=8.4