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

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 .

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.