X will equal 40.
EXPLANATION: since they’re parallel and these are opposite angles, add them together to equal 180.
x+(20)+3x=180.
add common variables.
4x+20=180
subtract 29 from both sides
4x=160
divide each side by 4 so isolate x
x=40
For the square with side length n, the diagonal measures:

<h3>
How to get the length of the diagonal?</h3>
The sidelength of the square is n, and we want to get the length of the diagonal d.
Notice that the diagonal is the hypotenuse of a right triangle whose catheti measure n.
Then we can use the Pythagorean theorem, which says that the square of the hypotenuse is equal to the sum of the squares of the cathetus;

That is the length of the diagonal.
If you want to learn more about right triangles:
brainly.com/question/2217700
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You use the Pythagorean theorem which is a^2+b^2=c^2
So....
.8^2+.6^2=c^2
.64+.36=c^2
1=c^2
1=c becaaue the square root of one is one
B and c are correct because you use divison and multiplication to get the right answer and the right answer is 99cm