Two times the sum of a number and twelve is 40. What is the number?

The random variable x represents the number of phone calls an author receives in a day, and it has a poisson distribution with a mean of 8.7 calls. what are the possible values of x

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25

Sole the equation 6n + 9 = -3n - 21 - 6 for n