It mentions that angle 1 is congruent to angle 2. They have the same angle measure. This allows us to replace angle 1 with angle 2 (because they're effectively the same thing).
We can say (angle 1) + (angle 3) = 90 (angle 2) + (angle 3) = 90 notice the replacement in bold
The second equation is an algebraic way of saying "angle 2 and angle 3 are complementary"
