Answer:
y = 3x
Step-by-step explanation:
Given y varies directly with x then the equation relating them is
y = kx ← k is the constant of variation
To find k use the condition when x = 7, y = 21
k = [tex]\frac{y}{x}[/tex] = [tex]\frac{21}{7}[/tex] = 3
y = 3x ← equation of variation
