Хорды AB и CD пересекаются в точке M, AB = 15 см, AM = 3 см и CM = DM. Найдите CD.

I seat on the chair
I work in (место работы тут)
I go to supermarket in the evening
I have English lessons every day
I try to remember everything
I am doing my homework
I do my exercises in the copybook
I usually do a lot of exercises in the class
I do many exercises at home
I have dictations once a week
I often speak English to my teacher 
I usually speak Russian