Ive got this equation system

{x+y= 0
{x^2 + y^2 = sqrt(125)
how do I solve it

I don't know why you have the { symbol in front of your two equations. I will ignore it.

The second equation is a circle about the origin with radius sqrt(125) = 11.18

The first equation can be rewritten y = -x and is a line through the origin with slope -1 (second and fourth quadrants).
The two lines intersect at x= -11.18 sin 45 = -7.90, y = +7.905, and
x= 11.18 sin 45 = 7.90, y = -7.905