I'm assuming you meant to say a^3. If so, then
a^3 = a*a*a
a^3 = 5*5*5
a^3 = 125
X+(x+1)+(x+2)+(x+3)+(x+4)+(x+5)=393
6x+15=393
6x=393-15
6x=378
x=63
third one is 63+2=65
It is B!!! If you take 3 3/4 times the # of sides which in this case is 4 equals 15 in the end.