Find the equilibrium index in an array where sum of elements to the left is equal to sum of elements to the right.

To find an equilibrium index, first calculate the total sum of all elements in the array. Initialize a leftSum to 0 and a rightSum to the total sum. Iterate through the array from left to right. In each iteration, subtract the current element from rightSum to represent the sum of elements to the right of the current index. Then, check if leftSum equals rightSum. If they are equal, the current index is an equilibrium index, and you should return it. After the check, add the current element to leftSum to prepare for the next iteration. If the loop completes without finding an equilibrium index, return -1.