What is the missing reason for line 8 in this proof?

this is the formula: v = the value of investment at the end of the time period

p = the principal amount (the initial amount invested)

r = the annual interest rate

n = the annual frequency of compounding (how many times a year interest is added)

t = the number of years the money is invested

^ means raise to the power of

the answer is b.

surface area: 169π

volume: 366π

i got it down.