Question

1. -9r + 10r

Explanation:

Step1: Combine like - terms

We have two terms with the variable $r$. Combine the coefficients.
$-9r + 10r=(-9 + 10)r$

Step2: Calculate the sum of coefficients

$-9+10 = 1$, so $(-9 + 10)r=1r=r$

Answer:

$r$