Solve
\(\mathsf{\dfrac{6}{15}=\dfrac{2}{c}}\)
Multiplying both sides by 15:
\(\mathsf{\diagup\!\!\!\!\! 15\cdot \dfrac{6}{\diagup\!\!\!\!\! 15}=15\cdot \dfrac{2}{c}}\\\\\\ \mathsf{6=\dfrac{30}{c}}\)
Since \(\mathsf{c}\) is a denominator of a fraction, it can't be zero. Then, you can also multiply both sides by \(\mathsf{c}:\)
\(\mathsf{6\cdot c=\dfrac{30}{\diagup\!\!\!\! c}\cdot \diagup\!\!\!\! c}\\\\\\ \mathsf{6\cdot c=30}\\\\\\ \mathsf{c=\dfrac{30}{6}}\\\\\\ \boxed{\begin{array}{l} \mathsf{c=5} \end{array}}\qquad\checkmark\)
Cross-multiplication works fine as well and you would get to the same result.
I hope this helps. :-)
