Let A be the Adenika's mark (=score);

let M be the Musa's    mark (score).


Then, according to the problem

       A - M = 19    (1)    ("In an examination adenike obtained 19 marks more than musa")

    1.5A - M =  6    (2)    ("If adenike had obtained one and a half time her own marks she would have scored 6 marks more than Musa’s marks")


It leads then, after subtracting equation (1) from equation (2) that

    0.5A = 6 - 19 = -13,  

or

       A = - 26.


So A is negative value.


Since it is IMPOSSIBLE, it means that something is WRONG with your initial / original formulation.


Look for the error in your writing.


Find the error, fix it, then re-post to the forum.