What i have learned in chapter 2.1 and the difficulties

What have i learned in chapter 2.1:
I have learned that, for e.g.:
a(x+y)=ax+ay
a(x-y+z)=ax-ay+az
The main thing is to multiply an algebraic expression by a single term. We can also extend it to find the product of two algebraic expressions such as (a+b)(x+y).

What are the difficulties?
I have difficulties solving the advanced questions such as: (2p-q)(2p+3q)-(p+q)(p-5q)
because i usually make careless mistakes and only 1 careless mistake will make the whole answer that i gave wrong.

What do i want to find out more?
I would like to know how to avoid making careless mistakes so that i will not get the question wrong and so get more marks for any examinations and tests that have questions that is linked to chapter 2.1 .