We can do this using the Pythagorean Theorem. It states that a² + b² = c²
Knowing the sides a and b to be 3/2, we plug them into the equation to find the diagonal.
(3/2)² + (3/2)² = c²
(3/2)² = 2.25
2.25 + 2.25 = c²
4.5 = c²
√4.5 = c
The diagonal is the √4.5, or as a decimal 2.12