Answer:
b1=7
Step-by-step explanation:
If we deal with geometric series, we should use the formulas for it. If there is the last term, it is not infinite geometric series. The sum of geometric series which isn't infinite is equal to b1(r^n-1)/(r-1) where b1 is the first term and r is the common ratio.
So 280= b1(3^n-1)/(3-1)
560= b1(3^n-1)
560= b1*3^n-b1
Then express bn=b1*r^(n-1)
189= b1*3^(n-1)
189= b1*3^n*1/3
567= b1*3^n when
560= b1*3^n-b1
560=567-b1
b1=7
