[tex]3m +  xy = \dfrac{xp}{4} [/tex]
Cross multiply:
[tex]4( 3m +  xy) = xp[/tex]
Distribute 4:
[tex]12m +  4xy = xp[/tex]
Take away 4xy from both sides:
[tex] xp - 4xy = 12m[/tex]
Take x as the common factor:
[tex] x(p - 4y) = 12m[/tex]
Divide both sides by p - 4y :
[tex]x =  \dfrac{12m}{p - 4y} [/tex]