I have to find the half-life of Iodine-129 using the regression equatioon

y=100e^-0.153x

The chart I was given is divided into age (billions of years) and % of original II-129:

2.0 (74%), 3.5 (59), 4.2 (53%), 4.3 (52%)

I know that to find the half-life I have to find the value of x. However, I am not sure how to do this. Could you help? Thanks.