A) You have products (I'm going to say items for future reference so you don't mix it up with multiplication) A, B, C, and D. Item A has weight 20 and volume 20, item B has weight 20 and volume 15, etc. Your combination of items will be A + B + C + D = sum of some quantity.
B) The system you're solving requires two conditions: that the total weight combined of 3 items, are to equal 220 lb -AND- the total volume of the 3 items is 125 cuft. Using the equations above, you have 2 equations and 3 variables. You are now able to write such an augmented matrix using the coefficients.
C) This one I believe you will know how to do once you find B.
D) Look at the values of A, B, and C you have for your system. You will have one parameter because you only have 2 equations for 3 variables. Now, be careful, since these are not just arbitrary numbers but represent physical quantities, you can't have negative quantities, so scale accordingly. In other words, there is a minimum and maximum "allowed" value for your numerical parameter.