To solve this problem, we start with the concept of joint variation. When we say that A varies jointly as B and C, it means that A can be expressed as:
A = k * B * C
where k is the constant of variation. In this case, we know that k = 3, B = 7, and C = 9. Now, substituting these values into the formula gives:
A = 3 * 7 * 9
Performing the multiplication:
A = 3 * 63 = 189
Thus, when B is 7 and C is 9, and the constant of variation is 3, the value of A is 189.