Are you sure that's all the information that the question includes? Usually they will give the height of the bridge or something similar, because both c) and a) are impossible to solve without knowing the height of the initial throw above the water.
Also, what level of math is this for? If you have taken intro calculus, I believe differentiation would really help here. We can find b) right now by taking the derivative of the function and setting it equal to 0 to find the maximum.
i.e. It takes the rock 1 second to reach maximum height.