public function multiplyBy($otherTerm)
 {
     $coef = $this->getCoefficient() * $otherTerm->getCoefficient();
     $newTerm = new self($coef, $this->getExponent() + $otherTerm->getExponent(), $this->term ? $this->term : $otherTerm->getTerm());
     if ($this->isNegative() != $otherTerm->isNegative()) {
         $newTerm->invert();
     }
     return $newTerm;
 }