Because each side of the cube is a square, you can find the area of one side, and multiply it by the number of sides on the cube.
The area of a square is
A = s², where s is the length of one side of the square
The length of one side is 5, so
A = 5²
A = 25
The area of one side of the cube is 25sq.cm.
Now, you multiply that by 6 sides.
25 x 6 = 150
The total surface area of the cube is 150 square centimeters.