I think your problem is supposed to be like this:
[tex] \frac{51}{2} = 2 \frac{2}{7} + \frac{n}{4} [/tex]
If so, then your first step should always be converting the 2 and 2/7 into an improper fraction:
[tex]\frac{51}{2} = \frac{16}{7} + \frac{n}{4}[/tex]
And then, you can solve for x as you normally would.
