To subtract normal random variables, you subtract the means and add the variances.
In symbols, N(a, u^2) - N(b, v^2) = N(a - b, u^2 + v^2).
C - P
~ N(0.1, 0.03^2 + 0.04^2)
= N(0.1, 0.05^2)
Select the following text if you want the rest of the solution:
P(piston doesn't fit)
= P(C - P < 0)
= P(Z < -0.1/0.05) where Z ~ N(0, 1)
= P(Z < -2)
= Phi(-2)
= 0.023