Uncertainty in radioactive half-life experiment

[ZedCar]

Homework Statement

I have conducted an experiment and found the gradient λ of a graph to be (2.15x10^-4) +- (0.15x10^-4)

The graph being ln(A) i.e. the number of disintegrations/unit time - dN/dt against time t

The half-life is
T(1/2) = ln(2)/λ = -3223.9

How do I find the uncertainty in the half-life?

Thank you!

Homework Equations

The Attempt at a Solution

 

[ZedCar]

I now realize that the % uncertainty in λ is 0.15/2.15 = 0.069

Therefore multiply T(1/2) of -3223.9 by 0.069 which equals ±224.9, which is the uncertainty.

3223.9 ± 224.9

How do I correct this to take account of significant figures?