Normal distribution with P(x=a) and P(x≥a) 
Given: height of $1000$ students normally distributed with $\mu=174.5\,\mathrm{cm}$, $\sigma=6.9\,\mathrm{cm}$
Find: a. $P(x<160\,\mathrm{cm})$ b. $P(x=175\,\mathrm{cm})$ c. $P(x\geq188\,\mathrm{cm})$

For a., I used the formula $z=x-\mu/\sigma$, which gives $-2.10$. Looking at the $z$ table, I derived $0.0179$.
However, for points b. and c. I cannot find any reference on how to solve "equal to" and "greater than or equal to" probabilities related to normal distribution. We only discussed $P(z < a)$, $P(z > a)$ and $P(a < z < b)$.

One of the properties of a continuous probability distribution like the normal distribution is that the probability of any individual outcome is zero.  This is true even if it is possible for that outcome to happen.
For example, it is possible that the height of a student is exactly 175 cm, but if you model height with a normal distribution, then the probability of that happening is zero.
Using this knowledge, you will find that $P(x\ge a)=P(x=a)+P(x>a)=0+P(x>a)=P(x>a)$.
Also note that if you were being asked for the probability, then your answer should be a number from $0$ to $1$.  (So your answer of $22$ is not a probability.)
