a) $5 x+2 y=21$

Step 1 ：We are given the linear equation \(5x + 2y = 21\).

Step 2 ：Since we only have one equation, we can only express one variable in terms of the other. Let's express x in terms of y.

Step 3 ：Rearranging the equation to solve for x, we get \(x = \frac{21}{5} - \frac{2y}{5}\).

Step 4 ：\(\boxed{x = \frac{21}{5} - \frac{2y}{5}}\) is the final answer.