The sum of the distances from point m to point (4,0) and point B (- 4,0) is 12. Find the trajectory equation of point M

The sum of the distances from point m to point (4,0) and point B (- 4,0) is 12. Find the trajectory equation of point M

Obviously, this is an ellipse
So C = 4
2a=12
a=6
Then B & sup2; = A & sup2; - C & sup2; = 20
The focus is on the x-axis
So x & sup2 / 36 + Y & sup2 / 20 = 1