x = 3
Looks like you have two similar Right triangles; a small one inside a larger one.
Their sides are proportional as follows
4/10 = 2x/(9 + 2x)
Cross Multiply
4(9 + 2x) = 10(2x)
36 + 8x = 20x
Subtract 8x from both sides of the equation
36 = 12x
Divide both sides by 12
3 = x
You can also set up the proportion as
10/4 = (9 + 2x)/2x
Cross multiplication will yield the same answer.
You can plug in x, to obtain 2x for the smaller triangle and 9 + 2x for the larger one. You can use the Pythagorean theorem to find any missing sides.
