Ineed on how to solve these only #17,19

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

The line plot below represents the number of letters written to overseas pen pals by the students at the waverly middle school.each x represents 10 students. how many students wrote more than 6 and fewer than 20 letters. a.250 b.240 c.230 d.220

Rewrite the expression in z to the n power. the expression i was given is: 1 over z to the negative 1 half power.