=FV(rate, nper, pmt, pv, type)
The periodic rate is in cell B6. The number of periods, nper, is in cell B8. The periodic payment, pmt, is in cell B3. You should use -B3 as the pmt argument since these payments represent cash outflow. The present value, pv, is zero. The type is also zero because the payments are made at the end of the period. Here is the appropriate formula:
=FV(B6,B8,-B3,0,0)
Since the last two arguments are both zero (the default value), you can also write this as:
=FV(B6,B8,-B3)