Answer:
x = 24
Step-by-step explanation:
Given that 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 y = 21 when x = 7, so
21 = 7k ( divide both sides by 7 )
3 = k
y = 3x ← equation of variation
When y = 72, then
72 = 3x ( divide both sides by 3 )
x = 24
