Notice that on the previous statement #6, the statement is:
m∠1 + m∠2 + 90 = 180 degrees
Then, on statement #7, it became:
m∠1 + m∠2 = 90 degrees.
We can infer that from Statement #6, Subtraction Property of Equality was applied to Statement #6 to be able to come up with Statement #7, that is:
m∠1 + m∠2 + 90 - 90 = 180 - 90
m∠1 + m∠2 = 90 degrees.
90 degrees was subtracted on both sides of the equation.
