Physics thinks in $\mathrm{m/s}$; the road signs outside think in $\mathrm{km/h}$. The bridge between them is exact and friendly: an hour is 3600 seconds, a kilometre is 1000 metres, and $3600 \div 1000 = 3.6$. So $1\ \mathrm{m/s} = 3.6\ \mathrm{km/h}$ — m/s to km/h multiplies by 3.6, km/h to m/s divides.

Never sure which way? The numbers themselves will tell you: for the same real speed, the km/h number is always the BIGGER one — a 10 m/s sprinter is doing 36 km/h. If your conversion made the km/h number smaller, you crossed the bridge backwards. Turn around and cross again.