a. What has 'infield' to do with anything? Please do not try to talk yourself into entirely unreasonble results. The maximum height cannot by UNDERground.
b. WAY too long. How did you arrive at this answer? Hint: -b/2a
c. Again, WAY too long.
d. Got this one.
Perhaps you are confusing the coefficients. The coefficient on is 'a' in the standard way of setting things up. Thus a = -4.9 m/