There are two ways we can find the minimum. The first is by noticing that this is an upwards parabola, so the minimum is at the vertex. For y = ax² + bx + c, the vertex is at x = -b/(2a).
x = -(-2.8) / (2 × 0.007)
x = 200
Alternatively, we can minimize C using calculus. Take the derivative with respect to x and set to 0:
which applies to a right triangle. I hope that helps you.