Factorise:

The given expression is $x^{2}+5 x-104$.

Find two numbers that follow the conditions given below:

Sum $=5$

Product $=-104$

Clearly, the numbers are 13 and $-8 .$

$x^{2}+5 x-104=x^{2}+13 x-8 x-104$

$=x(x+13)-8(x+13)$

$=(x+13)(x-8)$