How do you expand double brackets?

using the FOIL method is the best way to expand brackets.

FOIL stands for FIRST, OUTSIDE, INSIDE, LAST

For instance:

(a + 3) (2a + 5)

F (the first in each bracket multiplied together) - a x a = a²

O (the outside in each bracket multiplied together) - a x 5 = 5a

I (the inside in each bracket multiplied together) - 3 x 2a = 6a

L the last in each bracket multiplied together - 3 x 5 = 15

The 5a and 6a can be added --> 5a + 6a = 11a

Therefore the final answer would be a² + 11a + 15

I just wanted to correct one small mistake in Tara's comprehensive answer.

In the F line, it should be: a x 2a = 2a², so the final result would be 2a² + 11a + 15