to rotate 90 degrees counterclockwise we must transform the points like this
[tex](x,y)\longrightarrow(y,-x)[/tex]and then invert the sign of the y-coordinate or the second coordinate of each point, so the total transformation is
[tex](x,y)\longrightarrow(y,x)[/tex]now, transform each point
[tex](0,4)\longrightarrow(4,0)[/tex][tex](0,1)\longrightarrow(1,0)[/tex][tex](2,1)\longrightarrow(1,2)[/tex][tex](2,4)\longrightarrow(4,2)[/tex]