$a\frac{m}{s}=a\cdot \frac{0,001km}{\frac{1}{3600}h}=a\cdot \frac{0,001}{\frac{1}{3600}}\frac{km}{h}=a\cdot 0,001:\frac{1}{3600}\frac{km}{h}=a\cdot 0,001\cdot 3600\frac{km}{h}=a\cdot 3,6\frac{km}{h}=3,6a\frac{km}{h}$