If x + y = 7, then the value of x³ + y³ + 21xy is?

$$\eqalign{ & x + y = 7 \cr & \left( {{\text{cubing both sides}}} \right){\text{ }} \cr & \Rightarrow {\left( {x + y} \right)^3} = {\left( 7 \right)^3} \cr & \Rightarrow {x^3} + {y^3} + 3\left( {x + y} \right)xy = 343 \cr & \Rightarrow {x^3} + {y^3} + 21xy = 343 \cr} $$