Lin gave 12 flyers to 3 people. She took the remaining flyers (90 - (12*3)) and divided then into groups of 3 to give to the volunteers. That would be 3(12) + 3(y) = 90 which is also 3(12+y) = 90.
For story 2:
The equation would be "3(y) + 12 = 90" and not "3(12+y) = 90"
