5x + 3y=4

5x-3y= 2

Using elimination method 

5x + 3y=4 →  5x + 3y=4

-[5x-3y= 2] → -5x +3y= -2

6y = 2 

y → 1/3 and x = 3/5

solving (25x\(^2\) -9y\(^2\))

25 * [3/5]\(^2\) -9 * [1/3]\(^2\)

25 * \(\frac{9}{25}\) - 9 \(\frac{1}{9}\)

9 - 1 = 8