Let the slope of L be k, then the direction vector of L can be expressed as (m, km), where m is a real number and m is not equal to 0.

Let's take m= 1, then the direction vector of l (set to n), n=( 1, k),

Because the inclination angle of the straight line L is acute, k >: 0,

The absolute values of the projections of the vectors OA and OB on the straight line L are equal, that is, (vector OA* vector n)/|n|= (vector OB* vector n)/|n| This is a bit fast.

So vector OA* vector n= vector OB* vector n is (1, 4) * (1, k) = (-3, 1) * (- 1, k).

Get 1+4k=3-k, so k=2/5.

This is correct.