Answer:
area = x(3x-5)=3x²-5x
area =112cm²
3x²-5x=112
3x²-5x-112=0
b²-4ac = (-5)²-4(3*-112)=1369
Δ=1369→strictly positive, the equation 3x²−5x−112=0 admits two solutions.
(-b-√Δ)/2a =(5-37)/6 =-16/3
(-b+√Δ)/2a =(5+37)/6 = 7
a length is never negative, ⇒x= 7
width =7cm
length = 3x-5=21-5=16cm