book Maths Puzzle Numerical Ability

(2a+4)(4a^2-3a+4)=

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I'm not understanding the elimination technique that my teacher is explaining. My problem is:
3x+3y-2z=-32
x-3y+z=11
-x-3y+3z=23

I don't understand how to substitute after
Can you help me solve this equation?

sqrt 2y+7 + 4 = y

The 2y+7 is in the radical sign
Thanks