How many degrees ina right angle

How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

7. if the value of x varies directly with the value of y, and x = 3 when y = 21. what is the valu y, and x = 3 when y = 21. what is the value of x when y = 105?

For f(x)=2x+3 and g(x)=x^2- 8 find (f+g) (x)