Answer:
The number of hours.
Step-by-step explanation:
The dependent variable is what you are mesuaring.
Commutative property of multiplication is two numbers being multiplied either way,example 1x2 = 2x1.
In order to do this you have to substitute b with 9 and get 3a+9=54, now subtract 9 from both sides to leave a by itself and get 3a=45, now divide 3 in both sides to leave a by itself and get a=15
Hope this helps