Me find x and me maybe understand?

First, there are two similar triangles in the question. I draw the similar triangles on my attachment.

Second, the ratio of corresponding side in similar triangles is the same. So you write it as
3/(x+5) = 5/(2x+5)

Third, to know the value of x, you need to solve the equation above.

For better understanding, look into my attachment. The answer is x = 10