Hints to solve following nonlinear equation please?

y=-4.9x^2+20x+10
9.8x+y=20

substitute y = 20 - 9.8x

into the first equation:

20 - 9.8x = -4.9 x^2 + 20x + 10

4.9 x^2 - 29.8x + 10 = 0

x = 5.725 or 0.356

and you can get y.

These are strange equations to "solve" since one is the equation of motion of a body, and the other is the equation of its velocity.