Let the admission price for adults be 'x' and that of children be 'y'
Equation Formation.
2 x + 3 y = 80............equation(i)
3 x + y = 99.............equation(ii)
y = 99 - 3 x
2 x + 3(99 - 3 x) =80
2 x +297 - 9 x = 80
- 7 x = -217
x = 31
y = 99 - 3x
y = 99 - 3×31
y = 99 - 93
y = 6
Answer ⇒ Adults = $31
Children = $6
Sorry can't help you with that...wish I could
29 kn are equal to 2900000 cn

because

is a set with the element 0, so it's not the empty set, while

is the empty set.
If you type the equation into y1 on your calculator and use the table graph you get coordinates that you can graph. ( 0,-9 ) ( 1,-6 ) ( 2, -3 ) ( 3 , 0 ) ( 4, 3 ) ( 5 , 6 ) ( 6 , 9 ), just graph those and that should answer your question :)