f(x) = ln(x-3) is continuous on [4,6] and differentiable on (4,6).
By the Mean Value Theorem, there is at least one number, c, in the interval (4,6) so that f'(c) = [f(6) - f(4)]/(6-4)
1/(c-3) = [ln3 - ln1]/[3-1]
1/(c-3) = 0.5493
c-3 = 1.82
c = 4.82