Please help me solve and convert this problem into a quadratic equation

The length of a poster is 10cm longer than its width. If the area of the poster is
1200cm squared, what is the width of the poster.

let breadth be x
then length = 10 + x

area = l * b

1200 = x * x+10
1200 = x^2 + 10x
0= x^2 + 10x - 1200
0= x^2 + 40x - 30x -1200
0= x(x+40) - 30 (x+40)
0= (x-30)(x+40)

x=30 or x=-40
we take x=30 because breadth cannot be negative

breadth= 30cm
length=40cm

let x be the width of the poster, then x+10 is the length.

to get the area you multiply length times width, so:

x(x+10)=1200

x^2+10x-1200=0

Let width = x
=> length = x+10

1200 = x(x+10) (Length X Width)
1200 = x^2 + 10x
x^2 + 10x - 1200 = 0