Given:
To find:
The missing value.
Solution:
Let x be the missing value.
We have,
Subtract 
from both sides.
Multiplying both sides by 5, we get
So, the missing value is 3.
Therefore, the correct option is C.
Answer:
x = 28 cm
Step-by-step explanation:
Given:
Area of link shaded regions = 84 cm²
Required:
The value of x (diameter of the semicircle/length of the rectangle)
Solution:
Diameter of the semicircle = 2r = x
Length of rectangle (L) = 2r = x
Radius of semicircle (r) = ½x
Width of rectangle (W) = radius of semicircle = ½x
Use 3.14 as π
Area of the link shaded regions = area of rectangle - area of semicircle
Thus:
Area of the link shaded regions = (L*W) - (½*πr²)
Plug in the values
84 = (x*½x) - (½*3.14*(½x)²)
84 = x²/2 - (1.57*x²/4)
84 = x²(½ - 1.57/4)
84 = x²(0.5 - 0.3925)
84 = x²(0.1075)
Divide both sides by 0.1075
84/0.1075 = x²
781.4 = x²
√781.4 = x
27.9535329 = x
x = 28 cm
You can eliminate 81 because we know 
The dot is on approximately 9.32 (my estimate) so I would square that number:
(9.32)² = 86.8624 so if you round this you get 87
3.66666666 but u can round it to the third place
