The over all order of the reaction with the rate law of rate=k[A][B] is 2 since the exponent for [A] is 1 and the exponent for [B] is 1 and 1+1=2.
for example the order for the rate law rate=[A]²[B] is 3 since 2+1=3 and the order for a rate law of rate=[A]²[B]³ the order would be 5 since 2+3=5.
I hope this helps. Let me know if anything is unclear.
