var equation = ["+2x", "+1y", "-1x", "+2y"];
var equationRegex = /[a-zA-Z].*/g;
var numberRegex = /\+([^;]*)[a-zA-z]/g; // Make this the right regex, should match the number part and the sign e.g +3.5

function sortEquation(a, b) {
  var valueA = a.match(equationRegex)[0];
  var valueB = b.match(equationRegex)[0];

  if (valueA < valueB) {
    return -1;
  }
  if (valueA > valueB) {
    return 1;
  }
  // a must be equal to b
  return 0;
}


function reduceEquation (accumulator, currentValue, i, array) {
    var prevValue = array[i-1];
    var splitCurrentValue = currentValue.match(equationRegex);
    var splitPreviousValue = prevValue.match(equationRegex);

    var splitCurrentNumber = currentValue.match(numberRegex);
    var splitPreviousNumber = prevValue.match(numberRegex);

    if(splitCurrentValue[0] === splitPreviousValue[0]) {
        var total = Number(splitCurrentNumber[0]) + Number(splitPreviousNumber[0]);
        var signedTotal = total > 0 ? "+" + total : String(total);

        return signedTotal + splitCurrentValue[1];
    }

    return accumulator;
}

var finalAnswer = equation
    .sort(sortEquation)
    .reduce(reduceEquation);

console.log(finalAnswer + " = 0");