Probably you are supposed to compute
[tex]\displaystyle\lim_{x\to4}\frac{x^2-16}{x^2-4x}[/tex]
Factorize the numerator and denominator, then simplify:
[tex]\displaystyle\lim_{x\to4}\frac{(x-4)(x+4)}{x(x-4)}=\lim_{x\to4}\frac{x+4}x=\frac{4+4}4=2[/tex]
