There are many methods for expanding double brackets.
One of them is called FOIL. Each letter stands for the terms we will be multiplying out.
F - First
O - Outer
I - Inner
L - Last
Remember to include the signs in front of each term when multiplying them out.
After we have multiplied each of those, we are going to simplify the expression by collecting like terms.
Here's an example. Expand and simplify (x + 5)(x + 6).
F: x × x = x²
O: x × +6 = +6x
I: +5 × x = +5x
L: +5 × +6 = +30
Now let's simplify by collecting like terms (highlighted):
x² + 6x + 5x + 30 = x² + 11x + 30