Why is is that all of these fricking answers are wrong?

How to find the exponential function y=ca^x, with points (1,2) and (2,1)

The distribution of the amount of money spent by students for textbooks in a semester is approximately normal in shape with a mean of $235 and a standard deviation of $20. according to the standard deviation rule, how much did almost all (99.7%) of the students spend on textbooks in a semester?

What is the sum of the first 8 terms of the geometric series