What is the equation

  -12 - 3 = -15

Explanation:

We start at -12 which goes in the first box.

Moving to the left 3 units means we subtract off 3, so that's why 3 goes in the second box.

The arrow is pointing at -15 as the final destination, which goes in the final third box.

δabc ≅ δdcb by sas because

db ≅ ac . . given (side)

bc ≅ bc . . reflexive property (side)

∠dbc ≅ ∠ acb . . alternate interior angles where transversal bc crosses parallel lines bd and ac (included angle)