Explanation:
 a. separate the variables
 
 = 500-y
 dy/(500-y) = dx
 b. integrating your equation in part a to find the general equation of 
 differential
 Integrating on both sides
 
dy/(500-y) = 
dx
 -㏑(500-y) = x +C ..............(1)
 where C is constant of integration
 c. If y(0) = 7
 putting in equation (1)
 -㏑(500-7) = 0+C
 C = -㏑493 
 d. The particular solution is
 -㏑(500-y) = x -㏑473
 ㏑473/(500-y) = x
 473 = (500-y)
