Linearity property 4.3.1
Aperiodic signal <===> Fourier transform
$ \mathcal{F} ${ax(t)+ by(t)} <=> aX(jw) + bY(jw)
An example of this property can be used: x(t) = 3u(t) + 2u(t-5)
$ \mathcal{F} ${x(t)} = $ \mathcal{F} ${3u(t) + 2u(t-5)} = 3$ \mathcal{F} ${u(t)} + 2$ \mathcal{F} ${u(t-5)}
As you see the constants in front of those step functions are then placed in front of $ \mathcal{F} $ and this reflect on the linearity property so it hold true.
