Hi
First dont panic. this one is pretty easy.
you known that the perimeter is the addition of every sides of a figure.
you have the total perimeter : 84.6
so now you just have to withdraw every side you have to the total , and the missing measure will be the result.
have a try ! you can make it.
It is 20 because if u divide 78 by 3.75 ( 3 3/4 ) than equals 20.8
X² - 5x - 14 = (x - 7)(x + 2)
