The scenario is 'a ball is thrown upward with a speed of 48ft/s from the edge of a cliff 432 ft above the ground. Find its height above the ground t seconds later. When does it reach its maximum height? When does it hit the ground?'
In this case, the answer says the acceleration is negative and we have a(t) = -32. However, I don't understand where we should get this information from. is this some constant that physicist knows?
Yes, it is just a constant from physics: $a(t)=-32$ feet/$s^2$ comes from gravitational acceleration.