[tex]x:7=12:21\to\dfrac{x}{7}=\dfrac{12}{21}\ \ \ |cross\ multiply\\\\21x=7\cdot12\\21x=84\ \ \ |divide\ both\ sides\ by\ 21\\\boxed{x=4}\\\\Other\ method:\\\dfrac{x}{7}=\dfrac{12}{21}\\\\21:7=\boxed3\to12:\boxed3=4\\\\\boxed{\frac{4}{7}=\frac{12}{21}\to4:7=12:21}[/tex]
