Verify that: cos(x+y)cos(x-y)=cos^2x-sin^2y

using trig idenities, i can get to: cos^2xcos^2y-sin^2xsin^2y but i can't figure out how to get rid of sin^2x and cos^2y. anybody?

Suppose a laboratory has a 30 g sample of polonium-210. the half-life of polonium-210 is about 138 days how many half-lives of polonium-210 occur in 1104 days? how much polonium is in the sample 1104 days later? 9; 0.06 g 8; 0.12 g 8; 2,070 g