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.