Explanation:
It's saying x = 7 and y = 4, which means everywhere there is an x, you'll place a 7 and everywhere there is a y, you'll place a 4
x + 2y turns into (7) + 2(4)
Now we can answer the bottom question
2(4) = 8
7 + 8 = 15
so...
If x =7 and y=4, then x+2y= 15