expand (4a + 3b)(2a - 5b)?

Use FOIL Method.

It stands for
F - first to first
O - outer to outer
I - inner to inner
L - last to last

It means that you will multiply the "paired" terms.

(4a + 3b)(2a - 5b)

4a(2a) + 4a(-5b) + 3b(2a) + 3b(-5b)
8a^2 - 20ab + 6ab - 15b^2

ans: [ 8a^2 - 14ab - 15b^2 ]

8a² - 20ab + 6ab - 15b²
8a² - 14ab - 15b²

8a*2 + 6ab - 20ab - 15b*2 = 8a*2 -14ab - 15b*2

the *2 means squared

the answer is
-14ab+8a^2-15b^2

hold on plz
use f.o.i.l. and combine like terms:

8a-14ab-15b^2

(4a + 3b)(2a - 5b)
= 4a x 2a + 3b x 2a - 4a x 5b - 3b x 5b
= 8a^2 + 6ab - 20ab - 15b^2
= 8a^2 + (-14ab) - 15b^2
= 8a^2 - 14ab - 15b^2