You start by distributing the 5x, so that it’s 5x(2x) and 5x(3y). That becomes 10x^2 and 15xy. Put it back, and it’s 10x^2 - 15xy.
Expand
5x × 2x + 5x × -3y
Rake out the constants
(5 × 2)x + 5x × -3y
Simplify 5 × 2 to 10
10x + 5x × -3y
Use the product rule: x^ax^b = x^a + b
10x^2 + 5x × -3y
Simplify 5x × -3y to -15xy
= 10x^2 - 15xy
