How to use these formulas ($ arvores_repor: [volume] * 6;) and ($ total_pay: [number of trees]) in the code to make a calculation?

<?php


class  cal{

private $avore_repor;
private $volume;
private $valor_pagar;


public function setArvores_repor($r){
    $this->arvores_repor = $r;

}

public function getArvores_repor() {
        return $this->arvores_repor;

}

public function setVolume($v){
    $this->volume = $v;

}

public function getVolume() {
        return $this->volume;

}

public function setValor_pagar($p){
    $this->valor_pagar = $p ;

}

public function getValor_pagar() {
        return $this->valor_pagar ;

}

public function cal(){

}


}
?>