text: function derive(coefficient,exponent) {
    console.log( `${coefficient * exponent}x^${exponent -1}` )
    return `${coefficient * exponent}x^${exponent -1}`
  }
  
  // function tests
  
  derive(7,8)  // "56x^7");
  derive(5,9)  //  "45x^8");
  