A geometric series has first term a and common ratio r where r\geqslant 1

The sum of the first four terms of this series is 15 times the first term.

a) show that r^3 + r^2 +r -14=0

and having S4 = 15a

I've done the first part

But I don't get b

b) Show that r=2 is the only real solution for this equation.

Every time I use Latex I get an error when I preview, has anything changed?