TeX source:

\frac{\text { signal out of } \mathrm{Y}}{\text { signal } \mathrm{x}}=\frac{\text { signal out of Y }}{\text { signal out of } \mathrm{U}} \cdot \frac{\text { signal out of } \mathrm{U}}{\text { signal } \mathrm{x}}=(2)(3)=6