Question 760593
You can use distributive property, and if you want, also the lattice method for multiplication which is much like ordinary multiplication.


Try this way:


______1__________5x
.
3_____3_________15x
.
-4x___-4x_______-20x^2


The lefthand column and the upper row are the terms of each binomial factor.  Add all the other terms and simplify.

{{{3+15x+(-4x)+(-20x^2)}}}
{{{3+11x-20x^2}}}
or if you prefer,
{{{-20x^2+11x+3}}}