In Exercises $13-22,$ find the limit of each rational function $($a$)$ as $x$ and $($b$)$ as $x-.$ Write prop or $-$ where appropriate.

$13.f(x)=\frac{2x3}{5x7}$

$14.f(x)=\frac{2x^{3}7}{x^3-x^{2}x7}$

$15.f(x)=\frac{x1}{x^{2}3}$