3 questions in intro to mathematic modeling

Could you help me to finish question 10 in the first picture and question 4 and 5 in the following pictures.

three questions in total.

And for question 4 there is an extracredit （c）part：try to envision what kind of equilibrium point will represent a “stale-mate” but not perish together？

please do the extracredit as well.