Je comprend pas cette exercice

1) P triangle = 2x+y
    P trapeze = 3x+y

2) 2x+y = 23
    3x+y = 30

y = -2x+23

3x-2x+23 = 30
x = 30-23
x = 7

3x+y = 30
21+y = 30
y = 9