If x : y = 3 : 4, then (7x + 3y) : (7x - 3y) is equal to?

$$\eqalign{ & x:y = 3:4 \cr & \frac{{\left( {7x + 3y} \right)}}{{\left( {7x - 3y} \right)}} \cr & = \frac{y}{y}\left( {\frac{{7\frac{x}{y} + 3}}{{7\frac{x}{y} - 3}}} \right) \cr & = \frac{{7 \times \frac{3}{4} + 3}}{{7 \times \frac{3}{4} - 3}} \cr & = \frac{{\frac{{21}}{4} + 3}}{{\frac{{21}}{4} - 3}} \cr & = \frac{{\frac{{21 + 12}}{4}}}{{\frac{{21 - 12}}{4}}} \cr & = \frac{{11}}{3} \,\,or\,\, 11 : 3 \cr} $$