This question can be solved using suvat equations, since the acceleration due to gravity is constant. First make a list of the variables given in the question;
s=0.5m
u=0ms-1 (since the card is released from rest)
v=3.1ms-1
a=?
We can then select a suvat equation containing these variables, which we can use to solve the problem. In this case we choose v2=u2+2as, as this contains only one unknown (a). Substituting the variables in, we get;
3.12=02+2a(0.5)
This simplifies to;
3.12=a
So the acceleration due to gravity measured in this experiment is 9.61ms-2.