When is subtraction always the first operation done in an expression? A Never
B When it follows addition in an expression.
C When it is the first operation in an expression, from left to right.
D When it is in parentheses in an expression.

last option

step-by-step explanation:

given that δdef is right and isoceles,

assuming that side df is the hypotenuse means that de an ef are the other 2 sides that intersect at 90° and are of equal length.

because they are of equal length, we can say that they are congruent

because they intersect at 90°, their slopes must be opposite reciprocals.

only the last option satisfies these conditions.