Respuesta :

In quadrilateral sum of all angles =360⁰.
m∠A+m∠B+m∠C+m∠D = 360
90+99+x²+90= 360
x²=81
x=√81
x=9
Answer is A.9.
MrGumby
The answer is A) x = 9

We know this because every four sided figure has angles that measure up to 360. So we can subtract all of the angles and then use the remaining angle to solve the problem. 

360 - 90 - 90 - 99 = 81. 

So with the remaining angle being 81, we can solve for x. 

x^2 = 81
x = +/-9 

Since +9 is the only answer that makes sense in this case, x = 9.