Question

x-y=15
x²-y²=75
x+y=? ​

Answer 1

(x+y)(x-y)=x^2-y^2
(x+y) * 15 = 75
(x+y) = 75:15
(x+y) = 5

Answer 2

$\it x-y=15\ \ \ \ \ (*)\\ \\ x^2-y^2=75\ \Rightarrow (x-y)(x+y)=75 \stackrel{(*)}{\Longrightarrow}\ 15\cdot(x+y)=75|_{:15} \Rightarrow x+y=5$