Unable to understand this calculation. How came "0.2881 + 0.1554"? Please explore
P(12 < x < 15) = P(-0.8 < Z < 0.4)
= P(-0.8 < Z < 0) + P(O < Z < 0.4)
= P(O < Z < 0.8) + P(O < Z < 0.4)
= 0.2881 + 0.1554 = 0.443

This problem is about finding the area under the normal distribution curve between Z=-0.8 and Z= 0.4. This is the same as finding the area from 0 to 0.8 plus the area from 0 to 0.4 -- do you understand why that is?

But how come 0.2881 and 0.1554?

You use a table of values for the area as a function of z-score. See for example:

45701

Note that the value for z=0.8 is 0.2881, and the value for z=0.4 is 0.1554.