Given that y is partly constant and partly varies as x when y=12,x=3 and when y=18,x=6.Find the law connecting the variable and also find the value of y when x=4

Answer 1

Given that y is partly constant and partly varies as x
implies we can just go with the standard
y = mx + b

using y=12,x=3, we get 12 = 3m + b
using y=18,x=6, we get 18 = 6m + b

subtract those two equations:
6 = 3m
m = 2

go back into 12 = 3m + b, sub in m = 2, and you can find b

now you have the complete equation, easy to find y when x = 4