Atoy helicopter begins flying across a field with a speed of 3 m/s and an acceleration of 0.8 m/s2. how long does it take the helicopter to reach a speed of 7 m/s?

From equations of  motion;
v = u + at, where v = final velocity, u = initial velocity, a = acceleration, t = time taken to reach the final velocity. 
Using the values given,

7 = 3 + 0.8*t=> 7-3 = 0.8t

4 = 0.8t
t = 4/0.8 = 5 seconds