In 2012 five u. s stamps cost $ 2.20 how much did seven stamps cost

$3.08

Step-by-step explanation:

First you have to find how much each stamp costs. So u take 2.20 ÷ 5 = 0.44.

Next you take 0.44 x 7 and you should get $3.08

let's call the solutions x₁ = (5 - 2√7)/3 and x₂ = (5 + 2√7)/3

then the equation is (x-x₁)(x-x₂) = 0

if you fill that in you get a lot of numbers which you can recognize as ax²+bx+c=0.

step-by-step explanation:

you can see that a=1 (there is just an x²)

b = (-5 -2√7 + 2√7 - 5)/3 = -10/3

c = -2√7)/3 ) * (-5+2√7)/3) = -1/3

so your answer is 3x^2 – 10x – 1 = 0