Hi everyone, I am not able to understand the calculations marked Red.
Please help me to understand these calculations?
Hey @Manu_Chaudhary , welcome to linear algebra. 
Do you understand how the a) part of the problem is solved?
I think it might help you to take a look at a introduction to linear algebra. Nothing too difficult, just some basic techniques. There are a lot of simple practical examples you can find online, too — for example, this one.