not being an economist it would seem the amount saved is

S = I - C ?

in which case the marginal propensity to save is ds/dI = 1 - dC/dI ?

Simplify C

C = 10 + .7 I -.2 sqrt(I)

so dC/dI should be fairly simple

But then again I am not an economist or for that matter never really studied the subject--