Assuming that x=y, ie that the curve of the rib is circular in nature, here is how I would do it. Draw a center line in the cylinder, using center snap from one end to the other. Then scale it from the bottom using a factor of two or three to make it extend up past the top of the red bit. Then draw a horizontal line from the mid point of the red bit, holding shift after picking the midpoint to constrain it to ortho mode, that is long enough to intersect with the center line you just drew for the cylinder. Now click and hold on the circle button with the little arrow on the bottom right corner of the icon, a flyout will appear with all the different options for drawing circles. Pick the one that has a center point, and two edge points shown and follow the prompts, using snaps pick the center at the intersection of the two lines you drew, one edge point using the quadrant snap to the cylinder, the other using the midpoint snap to the red bit. Type in “trim”, and pick the two lines for the trimming objects, hit enter or space bar, and then click on the circle on the unwanted part to trim it. Now you have the arc you need from the cylinder to the midpoint of the red bit. Draw another line using quadrant snap from the outside top of the blue bit to the bottom. Draw another line using midpoint snap from the top midpoint of the front of the red bit to the bottom back midpoint of the grey bit. Then draw a third line using endpoint snap picking the ends of the two lines you have just created. Now you have three lines, and the trimmed circle, that create the profile of the rib you are looking to create. Click on the two puzzle pieces icon “join”, and the pick the four elements to create a closed curve that defines the rib. Then type in “extrudeCrv”, type in “b” to enable both sides option while extruding, and “s” to have it create a solid, pick your closed curve, and type in half of the desired thickness of the rib, given that it is extruding in both directions. And then you are done!

Let me know if this is confusing, I will be happy to try and help clear it up. Good luck with your class.