First, you would divide #x# out of the whole expression to get #x(x^2+8x-20)#
You would then factor #x^2+8x-20#. To factor it, you need to find two numbers that added together equal #8# and multiplied together equal #-20#. These two numbers would be #10# and #-2#, giving us #x(x+10)(x-2)#