What is the common difference for the sequence shown below?

Akitchen floor is made up of tiles that are in the shape of the triangle shown there are 40 tiles on the kitchen floor what is the total area of the floor

Kylie explained that (-4x+9)^2 will result in a difference of squares because (-4x+9)^2 (-4x+9)^2 =(-4x)^2+(9)^2=16x^2+81

What are the soultion(s) to the quadratic equation 40- x^2=0