If 999x + 888y = 1332 and 888x + 999y = 555, then $$x^{2} - y^{2}$$ Is equal to

999x+888y = 1332 ......................................(1)

888x+999y = 555 ......................................(2)

subtract (1) &(2)

111x-111y = 777

divide the equation by 7

x-y = 7 ................................(3)

add (1) & (2)

1887x+1887y = 1887

divide the equation by 1887

x+y = 1 ...................................(4)

now

$$x^{2} - y^{2}$$ = (x-y)*(x+y)

put values from (3) & (4)

therefore, $$x^{2} - y^{2}$$ = 7*1 = 7