write an equation that represents the weights on the hanger.

Explanation:

Step1: Analyze left - hand side

There are 2 weights of value $a$ and 1 weight of value 1, so it's $2a + 1$.

Step2: Analyze right - hand side

There are 9 weights of value 1, so it's 9.

Step3: Set up equation

Since the hanger is balanced, we set the two sides equal. $2a + 1=9$

Answer:

$2a + 1 = 9$