Let's set up the two equations.
First let M = Montague and C = Capulet
1) Last year 4C + 6M = 100
2) This year 8C +12M = 200
The above is your system of equations with two equations and two unknowns (C & M).
The problem does not ask to solve this system but rather just set it up, I believe. If you do try to solve it, you'll find there are multiple solutions.