This is a question involving math rather than physics.
The path of the projectile is given by the equation y = 16x^2 + 80. The point at which the velocity of the projectile is parallel to the ground has to be determined.
Treating the ground as the x-axis, the velocity at a point is parallel to the ground when the tangent to the path at the point is is parallel to the x-axis. A line parallel to the x-axis has a slope equal to 0.
For a function y = f(x), the slope of of the line at any value of x is given by f'(x). Here, y = 16x^2 + 80
=> y'= 32x
Solving y' = 0 gives 32x = 0 or x = 0
The value of y at x = 0 is y = 16*0^2 + 80 = 80
The projectile has a velocity parallel to the ground at the point (0, 80)