The square root is

Just square the top and bottom of the fraction.

and

so we have 12/16 and then we reduce to 3/4
The measure of B’C’ is 18 inches
I thought this would be simple, as I'm familiar with algebra and not really "The constant of proportionality," but I will do my best.
So this said "Constant of proportionality," is referring to basically the answers for the equation when X equals certain numbers.
Make a table of different answers when you plug in X and you get the 'Constant of proportionality.'
y = 2.5x + 3
y = 2.5(1) + 3
y = 2.5 + 3
y = 5.5
Since we plugged in 1 for X and got 5.5 for Y, our input and output is (1, 5.5)
Replace X for a different value, and you will get a bunch of different numbers that will in essence be your function inputs and outputs. Make a table of these and you have your answer.
EXAMPLE -
-= x =- -= y =-
-= 1 =- -= 5.5 =-
-= 2 =- -= 8 =-
-= 3 =- -= 11.5 =-
-= 4 =- -= 13 =-
So there you have it. I hope this helps! If you have any further questions, don't hesitate to ask.
2/7 ( k+5/8) = 2 and 2/7
2/7 (k+5/8) = 16/7 |multiply by 7
2(k+5/8) = 16 |divide by 2
k+5/8 = 8 |subtract 5/8
k=8-5/8 = 7 and 3/8 = 27/8 = 7.375