Since a parabola is symmetric around the vertex the x coordinate of the vertex is midway between 0 and 21 which is 10.5 The y coordinate of the vertex is at the max height or 70 so can put the equation in vertex form

y= a(x-10.5)^2 +70

Can use the point (21,0) to solve for a

0= a(21-10.5)^2 +70

-70 = a(10.5)^2

a= (-70)/(10.5)^2

a= -.635 so equation is

7=-.635(x-10.5)^2 +70