x²+ 14x = -749
x²+ 14x +?² = (x+?)²,
Let's extend (x+?)² in English:
(x+?)² = square of the first (=x²) + TWICE the 1st time the second (14x)+?²
So TWICE the 1st time the second = 2.x.? = 14x and ? 14x/2x = 7
then :x²+ 14x +?² = x² +14x + 49
Since we have added 49, let's subtract 49 to keep the balance of the equation:
(x² +14x + 49) - 49 = (x+7)² - 49 = - 749
(x+7)² = 700
