Hi Nico H
Let x = Adult tickets
Let y = Senior tickets
According to the information given total ticket sales were 4935 and the number of senior tickets sold was 35 less than twice the number of adult tickets,
y = 2x - 35
You can use this substitution for y and get everything in terms of x and solve.
13x + 11(2x -35) = 4935
Hope this helps