if the area of this rectangle is 84 then what if there is a x+2 on the length and a x+7 on the width

We have ..... Length * Width = Area .....so.....

(x +2) (x + 7) = 84   simplify

x^2 + 9x  + 14  = 84       subtract 84 from each side

x^2 + 9x - 70   = 0        factor

(x - 5) (x + 14)  = 0

And serring each factor to 0, we have that x = 5  or x = -14

Reject the second solution....so.....the length = (5 + 2) = 7   and the width = (5 + 7)  = 12