Let G=number of general admission tickets and S=number of students tickets.
G+S=196 and 3G+2.5S=519.
G=196-S so we can substitute for G in the second equation:
3(196-S)+2.5S=519.
588-3S+2.5S=519, 0.5S=588-519=69 so S=69/0.5=138 and G=196-138=58.
58 general admission tickets and 138 student ID tickets were sold.