Answer
$f'(x)=20x^{3}+30x^{2}+3$
$f"(x)=60x^{2}+60x$
$f^{(3)}(x)=120x+60$
Work Step by Step
$f(x)=5x^{4}+10x^{3}+3x+6$
$f'(x)=5(4)x^{4-1}+10(3)x^{3-1}+3x^{1-1}+0$
$f'(x)=20x^{3}+30x^{2}+3x^{0}$
$f'(x)=20x^{3}+30x^{2}+3$
$f"(x)=20(3)x^{3-1}+30(2)x^{2-1}+0$
$f"(x)=60x^{2}+60x^1$
$f"(x)=60x^{2}+60x$
$f^{(3)}(x)=60(2)x^{2-1}+60x^{1-1}$
$f^{(3)}(x)=120x^{1}+60x^{0}$
$f^{(3)}(x)=120x+60$