Upper and Lower Sums for a Function F on a Rectangle

Upper and lower sums for a function f on a rectangle R, using n subdivisions on each side, are $\left( 3 n ^ { 2 } + 8 n + 24 \right) / n ^ { 2 }$ and $\left( 3 n ^ { 2 } + 7 n + 8 \right) / n ^ { 2 }$ respectively.Evaluate $\int _ { R } f d A$