Book Maths Puzzle Algebra

im having trouble solving this problem for my homework. the book says=
' Use linear combinations to solve the system.'
x-y=4
x+y=12

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Book Other Question

How to sulve a quadratic equation by completing the square.
2s^2+5s=3 the sides of his rectangled yard is 12'x9'. the pool is trisected in the rectangle. what is the perimeter of the pool?