Question 135114
Diagonals of a rhombus are perpendicular. So the intersection of the two diagonals creates 4 right triangles inside a rhombus. see http://en.wikipedia.org/wiki/Rhombus for a picture.

Given that one diagonal is 12 and all four sides are 10. Then we know that the 4 interior triangles have sides 6,y,10

You can use Pythagorean theorem to solve for y, or you can notice that the ratio of the triangles is 6:y:10 which is a variant of 3:4:5 right triangle. So y = 8.
And the other diagonal is 8+8 = 16