Your two stmts should be written as follows:

1. y = 2x + 54

2. x = 3y - 72.

Now, as you can see, the two equations form a system. This is why you need both to answer this question: having a system of two equations with two unknowns means that you can solve it to get a single set of solutions, which you can then use to figure out the percentages you need.

I'll give you an example why stmt 1 doesn't do the job (it's very much the same thing with stmt 2):
Take x = 1 and y = 56 - this means that the percentage you're looking for is 1/56 * 100 = 1.17%

Take x = 5 and y = 64 and the percentage of y that is x will be 5/64 * 100 = 7.81%

As you can see, very different outcomes, but they both stick to the restriction of y = 2x + 54. This is why you need that second equation for your system: each stmt by itself is insufficient.