Pretend that the first term is 21, and the fourth term is 126 <span>it's the same thing. </span> <span>126 =21 +3d </span> <span>105=3d </span> <span>d=35 </span> <span>21 =a1 +6(35) </span> <span>21=a1 +210 </span> <span>a1=-189</span>
It is D because they cannot be negative feet away and if the fisherman is 12ft above water and the diver is 40ft below than 40 + 12 = 52. Hope this helps.