tolong dibantu ya teman teman

11)
x + y = 12 (i)
x - y = 4 (ii)

(i) + (ii)
2x = 16
x = 8

x + y = 12
8 + y = 12
y = 4

x^2 - y^2 = 8^2 - 4^2 = 64 - 16 = 48 (D)

12)
3x + y = 9000 ---> 6x + 2y = 18000 (i)
2x + 2y = 8400 ---> 6x + 6y = 25200 (ii)

(ii) - (i)
4y = 7200
y = 1800

3y = 3.1800 = Rp5.400,00 (E)

13)
x + y = 15 ---> 4x + 4y = 60 (i)
4x + 3y = 55 (ii)

(i) - (ii)
y = 5 (A)