You are given the vertices of
the ellipse, which are eight units apart on the y-axis, and centered on the origin. So you know that, in the ellipse equation:
. . . . .
...the value of "a" is 4, and (h, k) = (0, 0). You are given a point on the ellipse. Plug the x- and y-values from this point into the equation, along with the other known values, and solve for the one remaining value.