a = 7
ContourPlot[
      (x^2) (y^3) + 3 x (E^(-x y)), {x, -3, 
        5}, {y, -5, 10}, Contours -> {7}, 
      ContourShading -> False]

ContourPlot[
      x^2 y^3   +  3 x Exp[-x y],  {x,  -3,  
        3},  
{y,  4,  10},  Contours ->  {7},  
      ContourShading -> False]

ContourPlot[
      x^2 y^3   +  3 x Exp[-x y],  {x,  3,  
        10},  
{y,  -2,  2},  
      Contours ->  {7},  ContourShading -> False]