When solving an equation with two variables, replace the variables with the values given and then solve the now variable-free equation. (Remember order of operations, PEMDAS, Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.)

The perimeter of a triangle is the sum of the lengths of its sides:

p = x + y + z
p = 13cm + 5cm + 6cm = 24cm

The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

x² + 6x - 16 = 0
(x - 2)(x + 8) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (x - 2) or (x + 8) must equal zero:

If (x - 2) = 0, x must equal 2
If (x + 8) = 0, x must equal -8

So the solution is that x = 2 or -8

The surface area of a cube is (2 x length x width) + (2 x width x height) + (2 x length x height):

sa = 2lw + 2wh + 2lh
sa = (2 x 3 x 4) + (2 x 4 x 8) + (2 x 3 x 8)
sa = (24) + (64) + (48)
sa = 136

To multiply binomials, use the FOIL method. FOIL stands for First, Outside, Inside, Last and refers to the position of each term in the parentheses:

(y + 7)(y - 3)
(y x y) + (y x -3) + (7 x y) + (7 x -3)
y² - 3y + 7y - 21
y² + 4y - 21