Let y = safe load in pounds and x = length in feet of a horizontal beam. a constant of proportionality k exists such that . if a beam can hold 2,000 pounds at 15 feet, what is the safe load if the length of the beam is 10 feet?

Proportionality constant would be expressed by

y=kx

We know that at 15ft the beam can hold 2,000lbs. (15, 2000)

y=kx
2000=k(15)      = 2000/15=k     = 400/3=k. Now with the k solved equation looks like. 
y=400/3*x  
Plug in the 10 feet for x and solve for y.

y=400/3(10)
y=4000/3  
y=1,333.33 lbs