Construct a pro forma balance sheet that indicates the firm’s optimal capital structure. Compare this balance sheet with the firm’s current balance sheet. What course of action should the firm take?

The firm's current balance sheet is as follows:
A firm’s current balance sheet is as follows:

Assets $100 Debt $10
Equity $90

To answer the question, I believe the balance sheet should be as follows:
Assets $100 Debt $20
Equity $80
Total Assets $100 Total Debt & Equity $100

To answer the last part of the question, I would say that the company’s weighted-average cost of capital with the debt of 20% and the equity of 80% is the optimal capital structure. Therefore, the company should increase the debt weight from 0% to 20%. As a result, debt is increased from $10 to $20, and equity is reduced from $90 to $80.

AM I ON THE RIGHT TRACK WITH THIS? ARE MY CALCULATIONS CORRECT?

If you've determined that the optimal debt-to-assets ratio is indeed 20%, then you're on the right track. You might want to be more specific with a couple or three particular strategies the company might employ to move closer to the target structure. (A leveraged recap comes to mind, e.g. where the company issues debt and uses the proceeds to repurchase some equity shares.)

Also, proofread your answer for a typo--the debt's weight isn't currently 0%, it's...