How can you model the percent of a number

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

Which is a true statement comparing the graphs of x^2/6^2-y^2/8^2 = 1 and x^2/8^2-y^2/6^2 the foci of both graphs are the same points. the lengths of both transverse axes are the same. the directrices of = 1 are horizontal while the directrices of = 1 are vertical. the vertices of = 1 are on the y-axis while the vertices of = 1 are on the x-axis.

100 points, hi, i’m not sure what to do here,the data doesn’t seem to be quadratic .? can anyone me, in advance