Subtract $ \dfrac{x}{3} $ from $ \dfrac{y}{10} $ to get $ \dfrac{ \color{purple}{ -10x+3y } }{ 30 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 3 }$ and the second by $\color{blue}{ 10 }$.

$$ \begin{aligned} \frac{y}{10} - \frac{x}{3} & = \frac{ y \cdot \color{blue}{ 3 }}{ 10 \cdot \color{blue}{ 3 }} - \frac{ x \cdot \color{blue}{ 10 }}{ 3 \cdot \color{blue}{ 10 }} = \\[1ex] &=\frac{ \color{purple}{ 3y } }{ 30 } - \frac{ \color{purple}{ 10x } }{ 30 }=\frac{ \color{purple}{ -10x+3y } }{ 30 } \end{aligned} $$