Distance：18km
DepartureDelay : 5分
ArrivalDelay : 4分

Wolfram Language 14.0.0 Engine for Microsoft Windows (64-bit)
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In[1]:= f[Distance_:18,DepartureDelay_:5,ArrivalDelay_:4] := (
Velocity2 = Distance/(Distance/Velocity1 - DepartureDelay - ArrivalDelay);
Solve[x/Velocity1==x/Velocity2 + DepartureDelay,x]
)

In[2]:= f[]

Out[2]= {{x -> 10}}