Merci de m'aider rapidement !

I will answer in English.

We can prove that the angle APS is a triangle rectangle.

Remember that for a triangle rectangle of catheti A and B, and hypotenuse H, the Pythagorean's theorem says that:

A^2 + B^2 = H^2

In this case, we can assume that the hypotenuse is the longer side, AS, and the other two sides are the catheti.

Then we have:

H = 5x + 10

A = 3x + 6

B = 4x + 8

Now let's write the equation from the theorem, and let's see if its true.

A^2 + B^2 = H^2

( 3x + 6 )^2 + (4x + 8)^2 = (5x + 10)^2

So we can start with:

( 3x + 6 )^2 + (4x + 8)^2

And try to "transform" this into:

(5x + 10)^2

First, let's expand it:

((3x)^2 + 2*(3x)*6 + 6^2) + ( (4x)^2 + 2*(4x)*8 + 8^2)

9x^2 + 24x + 36 + 16x^2 + 64x + 64

25x^2 + 40x + 100

Now we can complete squares on the left side, by writing:

(5x)^2 + 2*10*(5x) + 10^2

(5x + 10)^2

Then we saw that the equation is true for every value of x, then we just prove that the triangle fulfills the theorem, thus, the triangle is a triangle rectangle.

y=mx+b is the final answer!   : )