What is an equation in standard form of an ellipse centered at the origin with vertex (-5 0) and co-vertex (0 -3?

The general equation of the ellipse is

where (h,k) the coordinates of the center 
a and b represents half the length of the axes 

given: center of the ellipse = (0,0)
vertex (-5 , 0) and co-vertex (0 , -3)
∴ (h,k) = (0,0)
∴ a = 5  and   b = 3
So, The equation in standard form of the ellipse is