If a square length and width decrease by a factor of 3. what would be the effect on the area of square

(Side note:  your image does not match your question.)

The effect on the area of the square would be to make it 0.49 times the area of the original square.

Decreasing by a factor of 3 is the same as taking 100% - 30%; s - 0.3s = 0.7s

The area would then be (0.7s)(0.7s) = 0.49s