Question

solve by multiplying (a+b)(2a+3b)(2x+y)

Answer 1

Multiply (a+b)(2a+3b) first:
(a+b)(2a+3b) = 2a^2 + 5ab + 3b^2.

Now multiply by (2x+y):
(2a^2+5ab+3b^2)(2x+y)
= 4a^2x + 10abx + 6b^2x + 2a^2y + 5aby + 3b^2y.

You can also write it as
a^2(4x+2y) + ab(10x+5y) + b^2(6x+3y)
or simply (2x+y)(2a^2+5ab+3b^2).