You have to FOIL.
(x + 2) (x - 4)
Multiply the value on the outside (left) of the first binomial by the value on the left of the second binomial.
x * x = x^2
Then, you take the the value on the outside of the first binomial again and multiply it by the second value in the second binomial.
x * -4 = -4x
Next, you take the second value of the first binomial (2) and multiply it by the first value in the second binomial.
2 * x = 2x
Now, you take the second value of both binomials and multiply them together.
2 * -4 = -8
Add all these values together and you will get: x^2 - 2x - 8