`O`, `A` and `B` are three points that are not in a straight line.

`vec(OA) = ul(a)` and `vec(OB) = ul(b)`.

The point `C` is such that `vec(OC) = <sup>3</sup>/<sub>4</sub>ul(a) + ul(b)`.

Another point `D` is such that `AB` and `DC` are parallel and of equal length.

`vec(OD) = sul(a) + tul(b)`

Find the value of the scalars `s` and `t`.