[tex]Dane:\\v_{o} = 54\frac{km}{h} = 54\cdot\frac{1000 \ m}{3600 \ s} = 15\frac{m}{s}\\t = 5 \ s\\v = 72\frac{km}{h} = 72\cdot\frac{1000 \ m}{3600 \ s} = 20\frac{m}{s}\\Szukane:\\a = ?\\\\a = \frac{\Delta v}{t}\\\\\Delta v = v-v_{o} = 20\frac{m}{s}-15\frac{m}{s} = 5\frac{m}{s}\\\\a = \frac{5\frac{m}{s}}{5 \ s}\\\\\boxed{a = 1\frac{m}{s^{2}}}[/tex]