The tub started with how many gallons of water f(x)=-3x=15

In this problem, the distributive property is very important, but there is another thing that we can look at to make this much easier. We notice that the terms on the left are being multiplied by 6. We can rewrite the right side so that it is also a multiple of 6:

6(3c - 5d + 6) = 6(1)

Now, we can divide both sides by 6, and we get:

6(3c - 5d + 6) = 6(1)

3c - 5d + 6 = 1

3c - 5d = -5