Step-by-step explanation: the two length 20 and 28 have a close range of value. So the next close range of value is the 38 inch which will make a perfect fit.
Note this rule is by trial and error method and no formula is behind it.
It is -11x + 24 because you multiply the numbers in the parentheses by 3 then collect like terms 10x +24 - 21x then you do 10x minus 24x and you get -11x+24
