Answer:
 15√2 N
Explanation:
The acceleration is given by ...
 a = F/m = 5t/5 = t . . . . meters/second^2
The velocity is the integral of acceleration:
 v = ∫a·dt = (1/2)t^2
This will be 9 m/s when ...
 9 = (1/2)t^2
 t = √18 . . . . seconds
And the force at that time is ...
 F = 5(√18) = 15√2 . . . . newtons