Answer:
6
Step-by-step explanation:
Given expression:
a varies directly as b;
a ∝ b
So;
a = kb where k is the constant of proportionality;
if a= 7 and b = 2;
7 = 2k
k = [tex]\frac{7}{2}[/tex]
Now, find b when a = 21
21 = [tex]\frac{7}{2}[/tex] x b
b = 6
