A car leaves a skid mark 72 m long, with decelaration being 6.00m/s2 what is speed before car braked?

firstly use U V A S T... V= 0, a= -6.0 S= 72... so then use V^2 = U^2 + 2as, when you put in the values you get 0 = U^2 - 864.. U = √864, So U = 29.4 (or in surd form, 12√6)