An airplane touches down on the runway with a speed of 70 m/s. It decelerates at a rate of -5 m/s2.

Answer 1

v = Vi + a t
Vi = 70 given
a =-5 given
so
v = 70 - 5 t

note: not 70 - (-5t)

Answer 2

a = -5 m/s^2
v = (-5t + k) m/s
when t = 0 , v = 70 m/s
70 = 0 + k ----> k = 70

v = (-5t + 70) m/s

now you can find the velocity at any time after touchdown
but you asked no question.
If the question was, "how long before the plane stops ?" , then
solve
-5t + 70 = 0 for t

Answer 3

v = vi + at
vi = 70 given
a = -5 given so that determines that
v = 70 - 5t

note:it is not 70 - (-5t)

If you write this down you teacher will not be disappointed, mine wasn’t.