Answer:
9
Step-by-step explanation:
3/7 multiplied by 21 = 3/7 x 21 = 9 = 3/7 x 21/1 = 63/7 = 9
Or if you can't do the math - Think of a pizza with 21 slices . Now cut this pizza into 7 equal parts. You will find that each part contains exactly 3 slices. Now, eat 3 of these parts. With three slices in each part, you ate 9 slices - which is three sevenths of 21.
In a right triangle with 45 degree angles the two sides are the same length. The hypotenuse is the length of the sides x sqrt(2)
The length of x = 3 and y = 3
This is the correct answer: 3x^3 - x^2 + 6x - 9

and surely you know how much that is.
Number four (or D) four feet