Patch Antenna Inset Feed Past Exam Question
I'm not sure on 3c either.  I get 10.02mm for 75.06 ohms.  And if you flip it to use 42.1mm I get 10.52mm for 75.06 ohms.  Perhaps there is something we are both missing.

For part 3d, I think they are referring to the spacing around the feed line.  In this image, not d but the cut out gap "g" along the feed. This is sometimes also called the "notch width".  Usually you can specify the gap as a ratio of the width of the feed line.  Where you can set g=W/10, W/15,..W/40...etc.



There are a number of ways to estimate the gap "g" for the design.  In general it effects the return loss's Q factor.  The smaller the gap the wider the S11 notch.

Here is an example from http://www.wseas.us/e-library/transactions/communications/2010/89-353.pdf



I'm not sure how your text or class is expecting you to estimate it without any design goals stated.  Perhaps they presented a typical estimate for "g" to center the S11 on your operating frequency?