Coefficient of linear expansion question

Dec 2013
Measurements of XRay scattering from an unidentified metal are made. The bragg peaks are θ = 50° and 46° for temperatures of 299K and 1250K respectively.

Calculate the linear expansion coefficient of the metal. This is given by

The rate of change isn't uniform, so the differential form of the Bragg equation must be used. This is
δd / d=δθ / tan θ.

I don't know how to start this question, could somebody please help?

Thanks in advance. :)