What is the missing reason in line 6

The equation x2 – 1x – 90 = 0 has solutions {a, b}. what is a + b?

Which is the focus of a parabola with equation y2=4x

Afootball quarterback has two more chances to throw a touchdown before his team is forced to punt the ball. he misses the receiver on the first throw 25% of the time. when his first throw is incomplete, he misses the receiver on the second throw 15% of the time. what is the probability of not throwing the ball to a receiver on either throw?