Question 282410
Using the fact that diagonals of a rhombus bisect each other and that they are also perpendicular, we have a right triangle with sides 12/2=6 and 4/2=2  with the hypotenuse being a side s of the rhombus.

By the Pythagorean Theorem we have then:

s^2 = 6^2 + 2^2
s^2 = 36 + 4
s^2 = 40
s = sqrt(40) 
s = sqrt(4*10)
s = sqrt(4)*sqrt(10)
s = 2*sqrt(10)