Xover 4 + 7 = 16, what does x equal?

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?

Can someone explain to me how to do

The measurement of one angle of a right triangle is 42° what is the measurement of the third angle? (there is no picture)