this is ocaml, there is an error inside,could you help me to find a it.

let mult_or_plus a b = if (three_or_ten x < 5) then (2 * ) else (2 + b)

//let mult_or_plus a b = if (three_or_ten x < 5) then (2 * ) else (2 + b) ^ This '(' might be unmatched ```

24 | let mult_or_plus a b = if (three_or_ten x < 5) then (2 * ) else (2 + b) Error: Syntax error: ')' expected