How do I solve this problem?

The length of a square is the same on all sides.

From the Pythagorean theorum we know that the length of a diagonal (C) is equal to the square root of the sum of the sides (A and B) squared:

[tex] {a}^{2} + {b}^{2} = {c}^{2} [/tex]

We also know that the area of a square is equal to the length of one of the sides squared.

[tex] {l}^{2} = a[/tex]

so to find the length of the diagonal we first need to find the length of a side. we do this using the area of the square Equation:

[tex] {l}^{2} = 196 \\ l = 14[/tex]

now we plug 14 into the Pythagorean theorum. because it's a square we know that a=b.

[tex]l = 14 = a = b[/tex]

plugging 14 into Pythagorean theorum:

[tex] {a}^{2} + {b}^{2} = {c}^{2} \\ {14}^{2} + {14}^{2} = {c}^{2} \\ 196 + 196 = {c}^{2} \\ 392 = {c}^{2} \\ c = \sqrt{392} [/tex]

simplify the √392 by breaking it up into factors and simplifying the largest perfect square.

1*392=392
2*196
4*98
7*56
8*49
14*28=392

These are all the numbers that multiply together to make equal 392 are called factors

Find one that has a perfect square. Here there are two.
4*98 and 8*49

pick one to simplify. it doesn't matter which. I'll choose 8*49

[tex] \sqrt{392} = \sqrt{8 \times 49} \\ \sqrt{8 \times 49} = \sqrt{8 \times {7}^{2} } \\ \sqrt{8 \times {7}^{2} } = 7 \sqrt{8} [/tex]

Notice this isn't one of our answers. That's because we can do this process again with the √8:

[tex]7 \sqrt{8} = 7 \sqrt{4 \times 2 } \\ 7 \sqrt{4 \times 2 } = 7 \sqrt{ {2}^{2} \times 2 } \\ 7 \sqrt{ {2}^{2} \times 2 } = (7 \times 2) \sqrt{2} \\ = 14 \sqrt{2} [/tex]

So C is the answer.