gistfile1.txt

// y = (x - xa) * (yb - ya) / (xb - xa) + ya

window.x = () => { // eslint-disable-line

  const [xa, ya] = [-63, 4868.00];
  const [xb, yb] = [-42, 4778.75];
  const a = (yb - ya) / (xb - xa);
  // const y = x * (yb - ya) / (xb - xa) - xa * (yb - ya) / (xb - xa) + ya;
  // const b = x11 * (y12 - y11) / (x12 - x11) + y11;

  const x_perpend = xa + 100;
  const y_perpend = ya - 1 / a * 100;
  console.log(a, x_perpend, y_perpend);
};