Surface area of a cube = \( 6 X \left(side\right)^{2} \)

Let the side of cube = 'a'

then its surface area = \( 6a^{2} \)

new side of a cube = 2a

then its surface area = \( 6 \left(2a\right)^{2} = 24a^{2} \)

Therefore, Percentage increase = \( \Large \frac{24a^{2}-6a^{2}}{6a^{2}} \times 100 \)

= \( \Large  \frac{18a^{2}}{6a^{2}} \times 100 = 300\% \)