exam Maths Puzzle Numerical Ability

PLEASE GIVE ME 5 EXAMPLES OF THE FF:
FINDING THE DOMAIN OF
*LINEAR FUNCTION
*QUADRATIC
*SQUARE ROOT FUNCTION
*RATIONAL FUNCTION

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exam Other Question
the question says solve: 2X+6 (under a square root)- X-6(under a square root)=3
My teacher got X=15 but I can not figure out how he came to that conclusion, could you help me please? Which equations is undefined

A) y=4

B) y=-2x

C) x=4

D) y=x+4