exam Maths Puzzle Numerical Ability

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?

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exam Other Question
Five and one half times x minus seven and one third equals nine and one half. what does this equal? PLEASE GIVE ME 5 EXAMPLES OF THE FF:
FINDING THE DOMAIN OF
*LINEAR FUNCTION
*QUADRATIC
*SQUARE ROOT FUNCTION
*RATIONAL FUNCTION