Question

the amount of water in a bathtube as it drains can be modeled by this equation with y representing the number of gallons of water in the tub and x representing the time, in minutes, that the tube has been draining y=-7.3x+39.7 after how many minutes will the tub have 17.8 gallons of water remaining

Answer 1

Since $y$ is representing the number of gallons of water in the tub, we need to replace $y$ with the number of gallons remaining in the tube to find the time. Fortunately, the problem is telling us  just that: 17.8 gallons. The only thing left is replacing that value in the equation, and solve for $x$ to find the time:
$17.8=-7.3x+39.7$
$7.3x=21.9$
$x= \frac{21.9}{7.3}$
$x=3$

We can conclude that after 3 minutes the tub will have 17.8 gallons of water remaining.