Question 44653
You are right about 12 being the common denominator for the first equation.
My working is shown below. Hopefully this example will help you in future questions of this type:
{{{2x/3 + 3y/4 =11/12}}}
{{{8x/12 + 9y/12 =11/12}}}
Rearrange to give y in terms of x:
{{{9y/12 =(11-8x)/12}}}
{{{9y =(11-8x)}}}
{{{y =(11-8x)/9}}} ......................(1)
Now the second part:
{{{x/3 + 7y/18 =1/2 }}}.............................(2)
Substituite equation (1) into equation (2) for y:
{{{x/3 + 7(11-8x)/(18*9) =1/2}}}
{{{x/3 + (77-56x)/(162) =1/2}}}
Rearrange to give x:
{{{x/3+77/162-56/162=1/2}}}
Rearrange to give x:
{{{x/3+21/162=1/2}}}
{{{x/3=1/2-21/162}}}
{{{x/3=81/162-21/162}}}
{{{x/3=60/162}}}
{{{x=180/162}}}
{{{x=10/9}}}.................................(3)
Substitute equation (3) into equation (1) to find y:
{{{y =(11-8(10/9))/9}}} 
{{{y =(11-80/9)/9}}} 
{{{y=19/81}}}
So the solution to the two original equations is x=10/9 and y=19/81.

I hope this helps.
P.S. I am trying to start up my own homework help website. I would be extremely grateful if you would e-mail me some feedback on the help you received to adam.chapman@student.manchester.ac.uk
Adam