If y = 15 + 5(x + 2), and x = 6, what is y?

Because we're told that x = 6, we can just rewrite the original equation but replace the x with a 6. So:
y = 15 + 5(6 + 2)
6 + 2 = 8, so we can rewrite this again as:
y = 15 + 5(8)
5(8) is just another way of writing 5 times 8, which is 40, so
y = 15 + 40
15 + 40 = 55, so y = 55