construct x =ab by using compass alone, if a and b are given segments. I found the problem in the book "What is mathematics?".

The following is a description of Mohr's constructions.(Macheroni problem)
  9) Find $x = ab$, if $a$ and $b$ are given segments.   

I found the question didn't give a segment denotes 1.I've no idea how to construct the x.  
Can anyone give me a hint?

Ratios are naturally expressed geometrically in terms of height/width of rectangles (or equivalently by slope of the diagonal).
Notice how $x = ab$ is equivalent to
$$
\frac{x}{a} = \frac{b}{1},
$$
which suggests the following construction.  (As mentioned in the other answer, this all depends on choosing a unit length to represent the number $1$.)

