An airplane starts at rest and excelerator down the runway for 20 seconds. at the end of the runway, its velocity is 80 m/s north. what is it’s acceleration?

According to the formula you have given us to work with . . .

1). The airplane's acceleration is

(80 m/s north - zero) / (20 sec) = 4 m/sec^2 north

2).  For the cyclist:

                                          (V-final - zero) / 20sec = 0.5 m/s^2 south

Multiply each side by 20s :  V-final = 0.5 m/s^2 south x (20sec) = 

                                                                                         10 m/s south

Less stable  hope this