Answer:
We will add 64 to both sides.
x=8±√20
Step-by-step explanation:
Here we are given the equation:

Now we take half of the coefficient of x and add it to x.
Coefficient of x is 16, half of 16 is 8.
Also we add square of 8 to both sides.
So new equation becomes:

Completing the square:

NOw let us simplify it to get the value of x.
Taking square root on both sides:

Bringing 8 to the right side:
x=8±√20
The third one should be your answer
The two triangular ends have area (3 cm)*(4 cm) = 12 cm^2, so the area of the rectangular faces is (204 -12) cm^2 = 192 cm^2. The sum of lengths of those rectangular faces is (3 cm +4 cm +5 cm) = 12 cm, so the width is given by
.. 192 cm^2 = (12 cm)*x
.. (192 cm^2)/(12 cm) = 16 cm = x
The 2nd selection is appropriate.