What Is the Correct Way to Calculate Unbalanced Force?

[sothis]

I am trying to work out question two on this page: http://www.bbc.co.uk/apps/ifl/scotland/learning/bitesize/higher/physics/quizengine?ContentType=text%2Fhtml%3Bcharset%3Dutf-8;templateStyle=physics;quiz=forces

m = 1500
a = 1.8
u=0
t_acc = 0.5
t_con = 8.5
t_res = 0.5

But I can't seem to get the right answer at all.

I have tried F=ma/t and searched for examples but I'm totally stuck.

 

[Dr.D]

Where did you get F=ma/t?

Perhaps you might show your work on this.