12C7xy{ }^{12} \mathrm{C}_{7} \frac{x}{y}12C7yx
12C7yx{ }^{12} C_{7} \frac{y}{x}12C7xy
12C6{ }^{12} \mathrm{C}_{6}12C6
12C7{ }^{12} \mathrm{C}_{7}12C7