In the applet below, please do the following:
1) Use the PARALLEL LINE tool to construct a line through C parallel to line f.
2) Use the SLOPE tool to measure and display the slopes of both lines. (They should be equal.)
3) If you want to see a grid, right-click and select "Show Grid" (or select the Grid option in upper right).
4) Use the PERPENDICULAR LINE tool to construct a line through D that's perpendicular to f.
5) Plot the point of intersection of line f and the line you just constructed in (4).
6) Verify the measure of the angle at which line f and the line you constructed in (4) = .
When you're done (or if you're unsure of something) feel free to check by watching the brief silent screencast below the applet.