Question 992567
Let adult tickets cost A and child tickets cost C. You are told
{{{3A + 4C = 121}}}
{{{2A + 3C = 86}}}

Multiply the first equation by 2 and the second by 3 to get
{{{6A + 8C = 242}}}
{{{6A + 9C = 258}}}
Now subtract the fourth equation from the third to get
{{{-C = -16}}}
Children cost 16 each

Use that to solve for A.
{{{3A + 4*16 = 121}}}
{{{3A = 57}}}
{{{A=19}}}