Multiply out 5x (2x-3y)

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