Take the numerator: (x/3)-6 is the same as (x-18)/3 and the denominator is (4+10x)/x.
Now the quotient: (x-18)/3÷(4+10x)/x=
(x-18)/3×(x/(4+10x))=
(x(x-18))/(3(4+10x)=
(x²-18x)/(6(2+5x)), or (x²-18x)/(12+30x).
We can take this further by using algebraic long division but that probably looks more complicated.