Stuck on problem solving quadratic equation question.

Hi there, struggling to understand how this question gets the answer.

So the area of the patio is 4m x 5m = 20m^2

The area of the whole thing would be (4 + x)(5 + x) = ? is the ? 20m^2 + 10m^2

Expanding that gives x^2 + 9x + 20 = ?

I'm sort of lost on how they get 4x^2 + 18x = 10

(4+2x)(5+2x) because theres an x on either side of the patio.

Doh!

Now I can see it .

Thanks.

-20 because of the patio itself