What is the missing numerator? 2/7 = n/21 a. 3 b. 5 c. 6 d. 8

C. 6

Step-by-step explanation:

2                  n

=

7                   21

We can use cross products to solve this.

2 * 21 = 7 * n

42 = 7n

Divide each side by 7

42/7 = 7n/7

6 =n

3. parallelogram

step-by-step explanation:

we turn each of our sides into direction vectors

ab=b-a=(3 - 7, 1 - 0) = (-4, 1)

bc=c-b=(1,6)

cd=d-c=(4,-1)

da=a-d=(-1,-6)

we see how the opposite sides have negated direction vectors; this is characteristic of a parallelogram.   we have congruent sides ab=cd and bc=da with unequal lengths because 1^2+6^2 is different from (-4)^2+1^2.   so it's not a square or a rhombus.  

we check the dot product of direction vectors of adjacent sides.   we have

ab dot bc = -4(1) + 1(6) = 2

that's not zero so the sides are not perpendicular.   we've ruled out rectangle.

the direction vectors indicate two pairs of congruent parallel sides, so we choose parallelogram over trapezoid.

thats not a system of equations, that is a solution