Hmm. Let's see. Any linear function is in the form of , with a non-zero a. Your exercise gave us two pieces of information about the velocity under time (let's call it ), namely:

and

So, replacing x by the appropriate values:

and

Two unknowns, two equations, solve the system and you get the values of a and b, giving you a model that describes the velocity of the object. Then use it to find the velocity at t = 3.5 s