Find x need it quick guys

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

You have a 32-foot fence around a square garden. there are 4 equal sections. you paint 13 of one section of the fence. what fraction of the fence did you paint? you have a 32-foot fence around a square garden. there are 4 equal sections. you paint 13 of one section of the fence. what fraction of the fence did you paint?

If square root x = -7, then x= -49 true or false