function doIt() {
          // gather variable data
          const nth = parseInt(document.getElementById("nth").value);
          const expr = document.getElementById("expr").value;
          const respectTo = document.getElementById("respectTo").value;

          const answer = `${math.format(
            nthDerivative(expr, nth, respectTo),
            14
          )}`;

          katex.render(answer, document.getElementById("nth-derivative"));
        }

        function nthDerivative(expr, n, respectTo) {
          let ans = math.derivative(expr, respectTo);

          for (let i = 0; i < n - 1; i++) {
            ans = math.derivative(ans, respectTo);
          }

          return ans;
        }