First of all we can do some estimation. 51% is near half so we're expecting an answer of around 900 days. Other than that we'd need the decay constant (except if the assumption I make later about the initial condition is valid)

Short Way

Half life is related to the decay constant by the following equation: where is the decay constant.

Long Way

Since you've posted this in differential equations we have the separable DE:

As said, this is separable:

(where is the constant of integration)

where (also a constant)

I am also going to assume the initial condition of when t =0 then (I'm not sure how valid this assumption is but without being told the decay constant it's the only way to find it)

Sub in the initial condition and you find that and rearranged it is

In your case:

Finally:

We can check this against what we'd expect and it's close enough to be correct. You'll need to sort out the attachment on your second one but try the same thing again.