If the value of y varies directly with x and y=-1 when x=3, find y if x=9

1. If y varies directly with x

Introduce the constant term k

2. y=kx

where k is the constant term

substitute the value of x and y to form the law

-1=3k

k=-1/3

:y=-1/3x ... is the law

To find the value of y when x is 9

Reinstate the law which is y=-1/3x and substitute the value of x to find y

y=-1/3*9

y=-3