Hey there! :)
First, let's substitute y with 0.8, and multiply by 8
0.8 × 8 = 6.4
Here is the new equation:
4x + 6.4 = 40
Now, subtract 6.4 from 40
40 - 6.4 = 33.6
That means 4x = 33.6
Divide both by 4
4x ÷ 4 = x
33.6 ÷ 4 = 8.4
x = 8.4
Hope this helps :)