$$h/2=3/N+5N-21$$
$h=10N-42+\frac{6}{N}$
$N\neq 0$
$N=\frac{\sqrt{h^{2}+84h+1524}+h+42}{20}$
$N=\frac{-\sqrt{h^{2}+84h+1524}+h+42}{20}$
$N=\frac{\sqrt{h^{2}+84h+1524}+h+42}{20}$
$N=\frac{-\sqrt{h^{2}+84h+1524}+h+42}{20}\text{, }h\geq 4\sqrt{15}-42\text{ or }h\leq -4\sqrt{15}-42$