A level Maths - Exponentials help

First i thimk you need to work out the gradient using a wider range of values maybe even plotting an actual graph. and then because t is negative perhaps you need to keep logb as positive so logb is 8/25 (or another more accurate value) and use that.

I did that and got my gradient as 0.3025 but I am still completely off the right answer which is M= 510 x 2^-t

Also I think where it has k = 510 that is a rounding of the orgiginal value of M when t=0 which ioO