First, find the y-intercept of the line y=-2x+7. When x=0 you have that y=-2·0+7, y=7, then point (0,7) is the y-intercept.
When you rotate this line 90° clockwise about the origin, then point (0,7) will be placed on the x-axis in positive x-direction. Rotation preserves the distances, then the coordinates of the image are (7,0).
