Tolong bantuin buat bsk

4a + 5b = -7 [×3]
3a + 2b = 12 [×4]

12a - 15b = -21
12a + 8b = 48
_______________ --
-23a = -69
a = 3

3a + 2b = 12
3(3) + 2b = 12
9 + 2b = 12
2b = 12-9
b = 3/2

a^2 - b^2
3^2 - (3/2)^2
9 - 9/4
36/4 - 9/4
27/4