What is the value of z?

(5z + 8) + (4z + 20) = 90

Well, you'd start off by combining the two like equations (so 5z + 4z will give you 9z and 8 + 20 will give you 28) so you can have 9z + 28 = 90. You'd subtract 28 from both sides of the equal sign and have 9z = 62. To get z from there you'd divide 62 by 9 and get 6.8888 ( ...) or rounded 6.89