Let's call the width . Since the length is three more than the width, it is
.
The area (which we know to be 70) is given by the multiplication of width and length:
To solve this equation, you can use the usual quadratic formula: given an equation , the two solutions are
which in your case becomes
So, two solutions are
Since we can't accept a negative length, we only accept the second solution.
So, the dimensions are 7 and 10