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We are supposed to find the value of z , which is equal to x y . To do this we have to find the values of x and y .
First, let us set up the equation x 2 + y 2 + x 2 − y 2 = 2 5 + 7 . This simplifies to 2 x 2 = 3 2 , then x 2 = 1 6 , so x = 4 .
Then let us solve for the equation 4 2 + y 2 = 2 5 . We get 1 6 + y 2 = 2 5 , then y 2 = 9 , and finally y = 3 .
Finally, let us plug 4 and 3 into the equation z= x y . We get z = 4 3 = 4 × 4 × 4 = 6 4 . So z = 6 4 .