Instead of using pythagorean formula, you can simply use concept of pythagorean triplets.
u slice the rhombus by the diagonals, you will have 4 congruent right
angled triangles, which in your case, it's 3cm of base n 4 cm of
height... therefore, finding the side of the rhombus is such as finding
the hypotenuse of the triangle. Just use 3,4,5 pythagorean triplets u'll
know that the length of hypotenuse (as well as the length of side of
rhombus) is 5 cm.