To find your answer you must substitute y for 0.8
4x + (8)(0.8) = 40
Now solve for x
====================================================================
SOLVING...
4x + (8)(0.8) = 40
4x + 6.4 = 40
Subtract 6.4 from each side
4x+ 6.4 − 6.4 = 40 − 6.4
4x = 33.6
Divide each side by 4
4x ÷ 4 = 33.6 ÷ 4
x = 8.4