Expand each of the following,using suitable identities: $(3a - 7b - c)^{2}$

(N/A) To expand $(3a - 7b - c)^{2}$,we use the algebraic identity: $(x + y + z)^{2} = x^{2} + y^{2} + z^{2} + 2xy + 2yz + 2zx$.
Here,$x = 3a$,$y = -7b$,and $z = -c$.
Substituting these values into the identity:
$(3a - 7b - c)^{2} = (3a)^{2} + (-7b)^{2} + (-c)^{2} + 2(3a)(-7b) + 2(-7b)(-c) + 2(-c)(3a)$
Calculating each term:
$= 9a^{2} + 49b^{2} + c^{2} + (-42ab) + (14bc) + (-6ca)$
Simplifying the expression:
$= 9a^{2} + 49b^{2} + c^{2} - 42ab + 14bc - 6ca$