Question 372754
Just like with fractions you need a common denominator. 
In this case, {{{(6x)}}}
{{{m(x)=(6x-9)((6x)/(6x))=(6x(6x-9))/(6x)}}}
So then 
{{{m(x)-k(x)=(6x(6x-9))/(6x)-(2-3x)/(6x)}}} 
{{{m(x)-k(x)=(6x(6x-9)-(2-3x))/(6x)}}}
 {{{m(x)-k(x)=(36x^2-54x-2+3x)/(6x)}}}
{{{highlight(m(x)-k(x)=(36x^2-51x-2)/(6x))}}}