apply Function.Injective.injOn
case hf
k : ℝ
hk : 0 < k
s : Set ℝ
hs : MeasurableSet s
⊢ Set.InjOn (fun u => k * u) s
case hf.h
k : ℝ
hk : 0 < k
s : Set ℝ
hs : MeasurableSet s
⊢ Function.Injective fun u => k * u