We want to solve the following system of equations 

and 

where x is the number of adult tickets sold and y is the number of child tickets sold. 
From the first equation, we can deduce that 

If we replace this value on the second one, we get 

If we operate on the left hand side, we get 

Then, if we subtract 14 on both sides, we get 

Finally, if we divide both sides by 3, we get 

If we replace this value in the original equation, we have that 

So 8 adult tickes and 7 child tickets were sold in total.