tolong nomer 3.......

x(a , b)--270°----> x'(b , -a)
B(2 , 1)--270°----> B'(1 , -2)

atau

|x'| = |0 1| × |x|
|y'|....|-1 0|...|y|

|x'| = |0 1| × |2|
|y'|....|-1 0|...|1 |

|x'| = |0×2 + 1×1|
|y'|....|-1×2 + 0×1|

|x'| = |0 + 1|
|y'|....|-2+0|

|x'| = | 1 |
|y'|....|-2|

B'(1 , -2)