Linear Kinematics

i have always been weak in maths , i am trying to solve the que will post soon as i will get the solution

Its more physics.
With s(t) = 0.5 * a * t² + v * t + s_0
Therefore you'll get v(t) = s'(t) = a*t+v
=> v(30) = 15ft/s² * 30 s + 30ft/s = 480ft/s (= 146 m/s)

b) Complete stop => v(t) = 0
So: v(3) = 15ft/s² * 3 s + 30 ft/s = 75 ft/s
=> v(t) = 0 = -30ft/s² * t + 75 ft/s
=> -30ft/s² * t = -75 ft/s => t = (-75 ft/s) / (-30ft/s²) = 2,5 s