Determine if the following limits exists or do not exist. If they exist find the limit$.$

$($a$)\underset{(xa)}{lim}\frac{3x^2-5y^2}{5x^2{y}^2}$

$($b$)\underset{(x,a)(-4)}{lim}\frac{x{y}^4}{x^2{y}^4}$

$($c$)\underset{(x,a)(Lin)}{lim}\frac{3y^{3}x}{x^4{y}^4}$