solve graphically.
x^2 + y^2 = 10
y = 3x
x^2 + y^2 = 10 is a circle with centre at (0,0) and radius √10
y = 3x is a straight line going through the origina with a slope of 3
It will cut the circle twice.
If you make a good graph you should see the intersections at (1,3) and (-1,-3)