In this prooof, we need to complete the 8th step to simplify the expression.
We are given:

Combining like terms on the left-hand side, we get

Subtracting the d squared term from each side, we have:

Next, we subtract the e squared term from each side:

Finally, we divide by -2 on each side:

Since d represents the slope of line AB and e represents the lope of line BC, we show that the slopes multiplied together give us -1.