As far as can see there are 2 correct methods shown, although there are other more efficient ways to solve this.
The 2 correct methods shown are:

and

The reason for this is because if we want to solve for "x", we must first isolate it. Normally we would isolate the "y variable", but because there is none in this equation we have to automatically assume that the "y" value has been given to us.
If you would like to see the math behind these 2 solutions I'll be more than happy to add it, if I can still edit my answer at that point in time. Also if you find out that this answer is wrong please tell me in the comments below, and I promise that I will come back and rework my math. Thanks!