If each unit needs 4 boards, and you have 58 boards, then to find the how many units you can make in total, you divide 58 by 4.
I did out the long division. 14 is the largest number of complete units Ann and Sue can make. If you would like to draw this out, you can draw a line for each board, and then circle every 4. You will find you have 14 circles, with 2 left over.
The length of the board is irrelevant, but I think it's what makes this problem seem complicated. You just need to divide the amount of boards for each unit (4) into the amount of boards they have available (58) the answer is 14.5. The .5 is not enough to make another complete unit, therefore, becomes irrelevant also. So the answer is 14. Now if your instructions tell you to draw a picture. You will need to draw 58 lines representing the boards. I would draw 4 lines across forming the bookshelf unit. You will see that you will have 14 units. If this is a school worksheet, sometimes they want you to truly understand the problem by drawing. As in earlier grades when learning to do addition, you would have to draw the 3 balls to add to the 6 balls and then draw nine balls. By drawing the images, it does make it easier to understand the division problem.