line-equation.js

function findLineEquation(point1, point2) {
  const slope = (point2.y - point1.y) / (point2.x - point1.x);
  const yIntercept = point1.y - slope * point1.x;
  return { slope, yIntercept };
}

function findCorrespondingY(point1, point2, x) {
  const lineEquation = findLineEquation(point1, point2);
  return lineEquation.slope * x + lineEquation.yIntercept;
}

const point1 = { x: 0, y: 0 };
const point2 = { x: 100, y: 100 };

console.log(findCorrespondingY(point1, point2, 50)); // 25