PLEASE HELP ME SOMEBODY!

What are “like terms”? why can we only add like terms?

Each person in an it department of 25 people works on at least one of 3 major projects: system security, program implementation, platform development. 10 people work on system security, 13 work on program implementation, and 12 work on platform development. 4 people work on system security and platform development, 5 people work on program implementation and platform development, and 2 people work on all three projects. how many people work on system security and program implementation?

1. evaluate 8x(2)y9(-2) for x=-1 and y=2 2. simplify the equation. -() 3. simplify the expression. [tex]\frac{5}{x(-2) y9(5)}[/tex]