Answer:
x = 15
Explanation:
given y varies directly with x then the equation relating them is
y = kx ← k is the constant of variation
to find k , substitute, x = 3 when y = 21 into the equation
21 = 3k ( divide both sides by 3 )
7 = k
y = 7x ← equation of variation
when y = 105 , then
105 = 7x ( divide both sides by 7 )
15 = x