You didn't state whether the rotation is clockwise or counter-clockwise,
so I'll do it both ways:

Here are the rules:

1. To rotate a point 90° CLOCKWISE about the origin,
Change the sign of the FIRST coordinate, then swap them.

2. To rotate a point 90° COUNTER-CLOCKWISE about the origin,
Change the sign of the SECOND coordinate, then swap them.

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If you want to rotate the triangle 90° CLOCKWISE, follow
rule 1:

D(-1,1) --> D'(1,1) 
E(5,-1) --> E'(-1,-5)
G(4,5)  --> F'(5,-4)



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If you want to rotate the triangle 90° COUNTER-CLOCKWISE, follow
rule 2:

D(-1,1) --> D'(-1,-1) 
E(5,-1) --> E'(1,5)
G(4,5)  --> F'(-5,4)




Edwin