Which is another way to write the expression r•(18-3)

The r variable outside the parentheses can multiply both the numbers in the parentheses.

This called distributing the r. We are using the distributive property.
The distributive property states that a(b + c) = ab + ac.

r(18 - 3) = 18r - 3r

So, (r * 18) - (r * 3) is the answer.