FindRoot[f, {xx0}]
searches for a numerical root of f, starting from x=x0.
FindRoot[lhs == rhs, {xx0}]
tries to solve the equation lhs == rhs.

FindRoot uses Newton's method, so the function of interest should have a first derivative.

FindRoot has attribute HoldAll and effectively uses Block to localize x. However, in the result x will eventually still be replaced by its value.

FindRoot stops after 100 iterations:

Find complex roots:

The function has to return numerical values:

The derivative must not be 0: