It is is a parallelogram, hence we have to face sides equal in length and the opposite angles are also the same. From the given above we have:
ab=14 and its opposite side cd=14
bc=20 and its opposite side da=20
Solving for the diagonal measurement bd, we have consecutive angles are equal to 180°
∠A+∠B=180°
∠A=180°-54°
∠A=126° , ∠B=54° ,∠C=126° and ∠D=54°
bd²=ab²+da²-2(ab)(da)cos126°
bd²=14²+20²-2*14*20cos126°
bd=30.42 unit
Solving for the angle dbc, we have
cos dbc=bc²+bd²-cd²/a*bc*bd
cos dbc=20²+30.42²-14²/2*20*30.42
dbc=21.76°