Ok to probe that ABD=CBD you need to prove that:
To prove that you have to use the Pythagorean theorem which states that given a rigth triangle abc with c as its hypotenuse:
In this problem AB and BC are the hypotenuses of triangles ABD and CBD respectively. So you have to use the theorem in both:
So in step 3 the type of statement would be:
And the reason would be: according to the pythagorean theorem
The last step would be step 4 and the statement would be:
The reason: they