Assume that the rate law for a generic chemical reaction is rate = [a][b]3 . what is the reaction order in a, the reaction order in b, and the overall reaction order?

If the reaction is
     X + Y → Products

Then the rate is
     R = k[X]ᵃ [Y]ᵇ

Where,
R = the rate of the reaction (mol L⁻¹ s⁻¹)
k = rate constant
[X] and [Y] = concentrations of the reactants (mol L⁻¹)
a = order of the reaction with respect to X
b = order of the reaction with respect to Y
overall reaction order, n = a + b

according to the given rate expression,
rate = [a][b]³

order of the reaction with respect to 'a' is 1
order of the reaction with respect to 'b' is 3

Overall reaction order = 1 + 3
                                       = 4