Can the sum or difference on two quadratic functions be a linear function?

Yes

Step-by-step explanation:

e.g if  x^2+y=7 and x^2+3y=9  are two quadratic functions, their difference is a linear function i.e. -2y=-2.

4 and 5)16.97 or 12√2

6)15.59 or 9√3

7)y = 15.59 or 9√3     x =27

8)17.80