Need some help with my homework!

Question

Question 7
a) Acceleration is the rate of change of velocity

Use your definition in (a) to show that v = u + at, where v is the final velocity, u is the initial velocity and a and t are the acceleration and the time interval respectively.

State the conditions that must be satisfied for the equation to be valid.

Question 11
An aircraft has a landing velocity of 50m/s and decelerates uniformly at 10m/s^2 until its velocity is reduced to 10m/s. Calculate
a) The time taken to slow down to this velocity, (answer is 4s but how do I get it?)
b) The distance covered during the deceleration. (answer is 120m but how do I get it?)

Thanks guys! :D