This is a question that can be solved with Pythagorean Theorem since it is a right triangle. The longest side, 29, is the hypotenuse and the sides 21 and x are the legs. So, the Pythagorean Theorem states,

Where,
c is the Hypotenuse, a and b are the legs. Putting the respective values gives us,

So, 20 is the side length of x.
ANSWER: 20
If you put them into to proportion
156/400 and then put a variable for the unknown x/1200
1200 divided by 400 = 3
156 times 3 = 468
468 order salad out of 1200
1 dozen = 12
144 / 12 = 12 dozen
12 dozen/ 3 = 4
They need 4 times the amount of flour:
3 1/4 x 4 = 13
They need 13 cups of flour
A6 = -8
a15= -62
There are (15 - 6) terms in between, therefore 9 terms in between.
-8 to -62 ... there is a difference of 54
54 ÷ 9 = 6
Therefore it gets -6 each term.
a6 = -8, a7= -14, a8 = -20 ......