To reflect a point over the line y = x you have to invert the coordinates of the said point, following the rule:
[tex](x,y)\to(y,x)[/tex]
For the point (10,0), the x-coordinate is 10 and the y-coordinate is 0. The x-coordinate of the preimage is the y-coordinate of the image, and the y-coordinate of the preimage is the x-coordinate of the image for the reflection:
[tex](10,0)\to(0,10)[/tex]
After the reflection. the coordinates of the image are (0,10)
