The slab is rectangle, so,

Area, A = 3x(x+4) = 63

3x^2 +12x -63 = 0

Divide both sides by 3,

x^2 +4x -21 = 0

Factoring,

(x +7)(x -3) = 0

x = -7 or 3 meters

Now I see what you mean by "in order for it to be paved"

The x = -7 meters cannot be measured because there are no negative measurements.

So, x = 3m

Therefore,

width = 3x = 3*3 = 9m.

lenght = (x +4) = (3 +4) = 7m.

Umm, again, therefore, width = 7 meters, and length = 9 meters. ---answer.