Question

simplify (3a+3b)^2

Answer 1

(3a + 3b)^2 = (3(a+b))^2 = 9(a+b)^2

Answer 2

use this
(a+b)^2=a2+b2+2ab

Answer 3

So you would have (3a+3b)(3a+3b)
You would have to FOIL everything out.
Fronts: 3a x 3a = 9a^2
Outers: 3a x 3b = 9ab
Inners: 3b x 3a = 9ab
Lasts: 3b x 3b = 9b^2

Now combine everything together:
9a^2+9b^2+9ab+9ab

Simplify 9a^2+9b^2+18ab.

There ya go!