The benighted naïve student will square 35 and 84, without dint of a calculator, add those two four-digit numbers, and then wonder how to take a square-root of the resultant ugly four-digit number. That approach is an unmitigated disaster. Instead, notice that 35 and 84 have GCM of 7: 35 = 7(5) and 84 = 7(12). This is simply the 5-12-13 right triangle, scaled up by a factor of 7. This means, x = 7(13) = 91. (NOTE: it's important to remember the 5-12-13 Pythagorean Triplet!)