Question

Can you help me with this question?

Answer 1

a geometric series looks like
\[ a + a r + a r^2 + a r^3 + ...\]
you are told
the sum of the first two terms i.e. a+ a *r add up to 90
and that the 3rd term is 24

Answer 2

in other words
\[ a + a r= 90 \\ a r^2 = 24 \]
one way to solve is to rearrange (by dividing both sides by r^2) the 2nd equation
\[ a r^2 = 24 \\ a= \frac{24}{r^2} \]

now replace "a" in the first equation by 24/r^2 (that is what the 2nd equation tells us)
\[ \frac{24}{r^2} + \frac{24}{r^2} \cdot r = 90 \]