解:$(1)$设$y_{1}=kx^2,$$y_{2}=\frac {m}{x-2}$
则$y=kx^2-\frac {m}{x-2}$
根据题意可得$\begin {cases}{4k-\frac {m}{-4}=3}\\{9k-m=13}\end {cases},$解得$\begin {cases}{k=1}\\{m=-4}\end {cases}$
则$y$关于$x$的函数表达式是$y=x^2+\frac 4{x-2}$
$(2)$当$x=5$时,$y=5^2+\frac {4}{5-2}=\frac {79}{3}$