The way you've written it
C(x) = 210 + 3.3x - 0.07x
which = 210 + 2.6x, combing the last two terms
C'(x) = Marginal Cost = 2.6 which means a flat horizontal MC curve, which isn't realistic, as if there were no diseconomies of scale.
BUT maybe you left out an exponent 2 on the last x term?
then C(x) = 210 +3.3x -0.07x^2 and
C'(x) = 3.3 -0.14x. this means a decreasing MC curve
trinomials tend to have descending or ascending exponents, so the last term was probably quadratic.