ENGAA diffraction help

So for the ENGAA Section 2 2017 4.c, they use the equation
Distance between maxima = (Dλ)/a.
I assume the distance between the maxima is the distance between the different orders and that diffraction could be a problem because at any points other than this the signal will be weaker, but what is a, is it the distance between the slits? Also, how would you derive the equation in the first place? Thanks in advance for any help.

Paper-
https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FAdmissions%2FENGAA%2FPapers%2FENGAA%25202017%2520Section%25202.pdf

Solutions-
https://www.physicsandmathstutor.com/admissions/engaa/solutions-2017/

From a quick scan, it looks like its the usual equation
d sin(theta) = m lambda
As theta is small, you can approximate sin(theta) with 1/1000 and m = 1 for the usual distance between maxima. I presume D is the 1000 (distance from receiver) and a=1 (distance between transmitters) so D/a gives 1/sin(theta), approximately.