Reduce this:

(2a+3b)^2 - 3b(4a+2b) - (2a+b)(2a-b)


I got this after I splittet it up:

(2a+3b)^2 = 4a^2 + 9b^2 + 12ab

-3b (4a + 2b) = -12ab -6b^2

-(2a+b)(2a-b) = - 4a^2 - 2ab + 2ab + b^2


so its:

4a^2 + 9b^2 + 12ab - 12ab - 6b^2 - 4a^2 - 2ab + 2ab + b^2 = 4b^2

4b^2 the answer I got.... is it right or wrong? :/

yes looks right to me

as a tip, pick something for a and b, then compute the result of your original expression and your final expression. if they don't match, it's wrong. if they do match, it's likely right (if you picked decent values), but still a chance it's not.