y = 7x^2 + 42x Put brackets around the first two terms
y = (7x^2 + 42x) Pull out the highest common numerical factor (7 in this case)
y = 7(x^2 + 6x ) Take 1/2 the 6 and square it put the result inside the brackets
y = 7(x^2 + 6x + (6/2)^2 ) subtract the (6/2)^2 outside the brackets. Remember to multiply by 7
y = 7(x^2 + 6x + (6/2)^2 ) - 7 (3^2)
y = 7(x^2 +6x + 3^2) - 7 * 9
y= 7(x^2 + 6x + 3^2) - 63 Express the bracket terms as a perfect square.
y = 7(x + 3)^2 - 63
The answer is D
