To combine like terms, add or subtract the coefficients (the numbers that come before the variables) of terms that have the same variable raised to the same exponent.

9a - 7a = 2a

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 1 x 3) + (2 x 3 x 4) + (2 x 1 x 4)
sa = (6) + (24) + (8)
sa = 38

An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite:

d° = b° + c°

To find angle c, remember that the sum of the interior angles of a triangle is 180°:

180° = a° + b° + c°
c° = 180° - a° - b°
c° = 180° - 21° - 61° = 98°

So, d° = 61° + 98° = 159°

A shortcut to get this answer is to remember that angles around a line add up to 180°:

a° + d° = 180°
d° = 180° - a°
d° = 180° - 21° = 159°

To multiply monomials, multiply the coefficients (the numbers that come before the variables) of each term, add the exponents of like variables, and multiply the different variables together.

(6a)(5ab) - (6a²)(3b)
(6 x 5)(a x a x b) - (6 x 3)(a² x b)
(30)(a¹⁺¹ x b) - (18)(a²b)
30a²b - 18a²b
12a²b

To solve this equation, repeatedly do the same thing to both sides of the equation until the variable is isolated on one side of the equal sign and the answer on the other.

4b - 4 = -3 + 3b
4b = -3 + 3b + 4
4b - 3b = -3 + 4
b = 1