So,
First, remember that the general equation of a hyperbola is given by the following:

In our problem, we're given that this hyperbola is centered at the origin, so C(h,k) = C(0,0). Then, h=0, and k=0.
Our equation can be written as:

The vertex of this hyperbola is set in the point (-7,0). We could use the following equation to find the value of a:

Where V is the vertex.
If we replace our values:

So,

Since "a" is squared in the equation, it doesn't matter if it is -7 or 7. The +/- sign indicates that there are two vertices at (-7,0) and (7,0) respectively.
Now, to find b, we could use the fact that the asymptotes of the hyperbola are:

Remember that these asymptotes have the form:

So, if we compare:

And, our equation will be:
