Question

In the formula vy=a t what is the value of a for an object in projectile motion

Answer 1

I THINK that formula gives the vertical speed of a projectile at the time
't' seconds after it's launched. 

(I can't really be sure of anything, because whenever you show a formula
to other people, you're supposed to tell them what each variable in it means,
and you didn't do that.)

If I'm on the right track, then the 'a' is the acceleration of gravity wherever
the projectile is flying.  If that happens to be on Earth, then the acceleration
is  9.8 meters per second² .