Absolutely correct . . . good work!
You are differentiating:
d^2 = 10x^2 + 14x + 7
with respect to x, which should give:
2d (d/dx)(d) = 20x^2 +14
so if (d/dx)(d) = 0 as we need for a stationary point you have:
20x^2 +14 = 0
which is what you have, but how did you get there?