Help I don't understand it

Rectangle Z is not a scale drawing of Rectangle Y.

This is because there is no equal sides where it makes up the constant proportionality.

6/4 (= 3/2) in the left and right sides of the rectangle (Y/Z)

4/3 in the top and bottom sides of the rectangle (Y/Z)

As you can see 3/2 is not equal to 4/3 of scaling, so therefore it is not a scaling.

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4