How do I calculate the independent effects of a $2\%$ increase in gross margin, a $2\%$ decrease in tax rate and a $5\%$decrease in sales using a base rate of $$2110$ in sales, cost of good sold of $1519,$ selling gen admin expenses of $295,$depreciation of $160,$ research and development of $49,$ total cost and expense of $2023,$ operating income of $87,$interest expense if $39,$ earnings before tax of $48$ provision for income tax of $10$ net income of $38$