Can help me explain for number 43

Using First Partial Derivatives In Exercises 41-44, find all values of x and y such that f,(x, y) = 0 and f,(x, y) = 0 simultaneously. 41. f(x, y) = x2 + 4xy + y2 - 4x + 16y + 3 42. f(x, y) = 3x3 - 12xy + y' 43. f( x, y) = + - + xy 44. f(x, y) = In(x2 + y2 + 1)