Find a value of (x-1)(y-1) when two real numbers x and y satisfy 2x = 5"=10.

A1 or just a as it is in the equations is just the initial term of the sequence.  In this case a1=1

r is the common ratio which is that constant ratio found by dividing any term by the term preceding it...

In this case r=3/1=9/3=27/9=etc=3

So a1=1 and r=3, C. is your answer.