Answer
$F(x) = 3x^{5/3} - 75$
Work Step by Step
$F(x) = \int 5x^{2/3} + C$
$F(x) = \int 5(\frac{3}{5}x^{5/3} + C)$
$F(x) = 3x^{5/3} + C$
Given $f(8) = 21$:
$F(8) = 3(8)^{5/3} + C$
$F(8) = 3(32) + C$
$21 = 96 + C$
$C = -75$
$F(x) = 3x^{5/3} - 75$
