Question 299455
Hint: If the given solution is (5,12), then this means that x=5 and y=12. Plug these values into the system to get 


{{{system(12=3(5)+a,12=-2(5)+b)}}}



and simplify to get 



{{{system(12=15+a,12=-10+b)}}}



You can see that you now have two equations in which you can directly solve for both 'a' and 'b'.