Can somebody PLEASE answer #11, #13, and #15. thank you sooo muchhh!!!

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)

If y varies directly as x2 and y=6 when x=6, find the constant of variation.

He weights of 2-pound bags of best dog food are approximately normally distributed with a given mean  and standard deviation  according to the empirical rule, what percentage of the bags will have weights within 3 standard deviations of the mean? 47.5%68%95%99.7%