Heres a word problem i'm having difficulty with.

Velocity of a projectile.The velocityvof a projectile is a linear function of timetthat it is in theair. A ball is thrown downward from the top of a tall building. It's velocity is 42 ft per sec after 1 sec and 74 ft per sec after 2 sec. Writevas a linear function oft.What is the velocity when

t =3.5 seconds?

this problem comes from the linear equation chapter.

i have this to work with

1. v = distance / time

2. t = 3.5 sec

3. m = y2 - y1 / x2 - x1

4. point slope form: y-y1 = m( x-x1 )

5. slope intercept: y = mx+b

but what i have to work with I still dont know how to come up with the soulution can someone please come up w/ the solution. This seems like a good problem to learn for a future physics class that i'll be required to take.