Ahyperbola with center at (0, 0), with x-intercepts at (4, 0) and (-4, 0), with b = 1, and opening horizontally has the equation

For a hyperbola, the traverse axis, where it opens at, in this case horizontally or the x-axis, is the one that has the positive fraction, in this case then, it will be the fraction with the "x" variable in it.

check the picture below, you can pretty much see from the x-intercepts, what the vertices are, and thus what "a" is.