I appologize if this is worded funny. It's been a while since highschool physics...
An object is thrown straight upward with an initial velocity of 1217 pixels per second. Due to the acceleration of gravity acting on the object, after 2 seconds the vertical velocity is 0 and the object is 1188 pixels above its starting point.
What is the value of the acceleration of gravity acting on the object?
The acceleration is the rate of change of velocity. If you are not too far from the Earth's surface, the acceleration due to gravity is almost constant. Let us suppose there are no other forces acting on the object.
Then in our case the acceleration is $-\frac{1217}{2}$ pixels per second per second. For the velocity changed by $-1217$ in $2$ seconds.
We used a negative number because you used a positive number for velocity,so presumably the up direction is the positive one.
Remark: It turns out that the distance travelled is not quite consistent with an assumption of constant acceleration. For if the only force acting on the object is gravity, then the net displacement after $t$ seconds should be $v_0 t+\frac{1}{2}at^2$, where $v_0$ is the initial velocity, and $a$ is the acceleration. In our case, if we calculate we find that the position should have been $1217$ pixels above the starting point.