Base = distance from O to C = x+1 units
Height = vertical distance from x axis to point B = x units
Area = Base*Height
42 = (x+1)*x
42 = x^2+x
x^2+x = 42
x^2+x-42 = 0
(x-6)(x+7) = 0
x-6=0 or x+7=0
x=6 or x=-7
Ignore the negative x value. It makes no sense to have a negative height.
The only practical solution is x = 6
If x = 6, then x+1 = 6+1 = 7
So the base is x+1 = 7 and the height is x = 6