The key idea here is that
speed of plane in wind = (speed of plane in air) + (speed of wind)
Let [tex]s[/tex] be the speed of the plane in still air, and [tex]w[/tex] the wind speed. A tailwind blows in the same direction as the plane, so for the first leg of the trip, we have
[tex]158\,\dfrac{\rm km}{\rm h}=s+w[/tex]
For the return trip, the wind would blow in the opposite direction:
[tex]112\,\dfrac{\rm km}{\rm h}=s-w[/tex]
Solve this system and you'll find [tex]s=135\,\dfrac{\rm km}{\rm h}[/tex] and [tex]w=23\,\dfrac{\rm km}{\rm h}[/tex].
