Answer:
x = 15
Explanation:
given that y varies directly with x
mathematically we can express this as
y = k x , where k is a constant
step 1: find k
given x = 3 and y = 21
y = k x
21 = k (3)
k = 7
hence the equation becomes
y = 7 x
when y = 105,
105 = 7x
x = 105 / 7 = 15