first order kinetics has the form of -d[A]/dt = k * [A]. Your formula is for second order kinetics.
then you can use the formula by solving this equation (or just look it up) and use the ratio [A](t) / [A](0) = 0,75 to find k. With k you can calculate the half life (formula can be found by taking [A](t) / [A](0) = 0,5)