Hey Inderpreet!
To find the area of a rectangle, you must multiply the length by the width.
A= L*w, L= 3w
A=3w*w
48 = 3w*w
48=3w^2
48/3 = w^2
16= w^2
squareroot of 16 = w
w= 4
plug 4 back in the equation of L= 3w
3(4) = 12
If the Area is equal to 48, and the length is three times the width, then the length must be equal to 12 inches, while the width is equal to 4.
Hope this helps!