There is a quick answer to this.....but let's solve it algebraically, first
Note 2sqrt(3) = sqrt(12)
So.....the remaining leg is
√ [ [sqrt(12)]^2 - [sqrt (3)]^2 ] = √ [12 - 3] = sqrt [ 9 ] = 3 units
Note that if the hypotenuse is 2√3 and the shortest leg is √3......then we have a 30 -60 -90 right triangle
The the angle opposite the hypotenuse = 90°
The angle opposite the shortest leg = 30°
So.....the remaining leg is opposite the 60° = √3 *√3 = 3 ...just what we found !!!!