Asquare wall tile has an area of 58,800 square millimeters. between which two measurements is the length of one side

As you know, all the four sides of a square has equal length 
To find the area of square you need this : A = a² (A- area and a is the lenght of sides).
As we do not know the length but we already have the area from the task, here is solution for one side of the square :
We just need to get the square root of the area:
A = a²
4220 = a²
√4220 = √a²
a = 64.9615271

Do hope you still need the answer because this one is 100% correct.