Determine the Constant of Variation for the Stated Condition $y$ Varies Jointly as a And

Determine the constant of variation for the stated condition.
- $y$ varies jointly as a and $b$ and inversely as the square root of $c . y = 18$ when $a = 2 , b = 9$ , and $c = 16$ . Find $y$ when $a = 6 , b = 7$ , and $c = 25$ .

A) $33.6$
B) $8.4$
C) $6.72$
D) 840