Hey there!! 
Always remember this : 
90-degree counterclockwise: (x, y) → (-y, x)
 180-degree counterclockwise: (x, y) → (-x, -y)
 270-degree counterclockwise: (x, y) → (y, -x)
1st quadrant : ( x , y ) 
second quadrant : ( -x , y ) 
Third quadrant : ( - x , - y ) 
Fourth quadrant : ( x , -y ) 
Now, the triangle abc lies in the 1st quadrant. So, all the vertexes will have positive values. 
Let's take the vertex  a as ( 1 , 2 ) 
Vertex b as ( 4 , 7 ) 
Vertex c as ( 5 , 9 )
If we rotate these values in 90 degress angle, these would become ( -y , x ) 
Vertex a : ( 1 , 2 ) ; After rotating : ( -2 , 1 ) 
Vertex b : ( 4 , 7 ) ; After rotating : ( -7 , 4 ) 
Vertex c : ( 5 , 9 ) ; After rotating : ( -9 , 5 ) 
Hope my answer helps!