Calculate the sustainable growth rate (SGR) for Company XXX, which has a return on equity (ROE) of 15%, a dividend payout ratio of 40%, and a retention ratio of 60%. Discuss the significance of the SGR as a measure of a company's ability to grow its sales and earnings without increasing its financial leverage. Analyze the factors that may constrain or facilitate sustainable growth and provide recommendations for achieving long-term financial stability and value creation.