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

y = - 3

Since y varies directly as x then

y = kx ← ( k is the constant of variation )

to find k use y = - 1 when x = 3

k = = = -

hence y = - x

when x = 9, then

y = - × 9 = - 3