If x and y are positive integers such that (3x + 7y) is a multiple of 11, then which of the followings is also a multiple of 11 ?

Answer: Option A

Let 3x + 7y = 11k
Then, y = $$\frac{(11k-3x)}{7}$$
Then,
$$\begin{array}{rl}& =5x-3y\\ & =5x-\frac{3(11k-3x)}{7}\\ & =\frac{35x-33k+9x}{7}\\ & =\frac{44x-33k}{7}\\ & =\frac{11(4x-3k)}{7}\end{array}$$
Which is divisible by 11