You can solve for the velocity and position functions by integrating using the fundamental theorem of calculus:
a(t) = 40 ft/s²
v(t) = v (0) + ∫₀ᵗ a(u) du
v(t) = -20 ft/s + ∫₀ᵗ (40 ft/s²) du
v(t) = -20 ft/s + (40 ft/s²) t
s(t) = s (0) + ∫₀ᵗ v(u) du
s(t) = 10 ft + ∫₀ᵗ (-20 ft/s + (40 ft/s²) u ) du
s(t) = 10 ft + (-20 ft/s) t + 1/2 (40 ft/s²) t ²
s(t) = 10 ft - (20 ft/s) t + (20 ft/s²) t ²