Surface area

Each face of the cube is a square of length $(x + 3)$

The area of each square,

$= (x + 3)(x + 3)$

$= x(x + 3) + 3(x + 3)$

$= x^2 + 3x + 3x + 9$

$= x^2 + 6x + 9$

Now,

The cube has six such faces

The total surface area of the cube,

$= 6 \times$ The area of one face

$= 6(x^2 + 6x + 9)$

$= 6x^2 + 36x + 54$

Area of a square: $(x + 3)(x +3) = (x + 3)^2 \color{#3D99F6} \text{ Using the identity } (x + y)^2 = x^2 + 2xy + y^2\color{#D61F06} \implies = x^2 + 6x + 9$

There are 6 squares so: $6 \times (x^2 + 6x + 9) \implies \color{#69047E} 6x^2 + 36x + 54$