Let the number of deer in the park is x and human visitors is y

Since each deer and each human has one head and given total heads are 39

=> x + y = 39  .................1

Again each deer has 4 legs and each human has 2 legs and total legs are 132

=> 4x + 2y = 132

=> 2x + y = 66   ................2                 {divide by 2 on both side}

Subtracting equation 1 from 2, we get

     2x - x = 66 - 39

=> x = 27

Put x = 27 in equation 1, we get

     27 + y = 39

=> y = 39 - 27

=> y = 12

Hence, there are 27 deer and 12 human visitors in the park.