Please provide detail explaination and include core concepts$/$ lessons as my teacher has not covered this. Thank you.

Give a right$-$linear grammar to generate each of the following languages:

$($a$)$ All strings over $\{a,b\}$ that contain exactly two b$\text{'}$s or exactly three b$\text{'}$s$,$ not more.

$($b$)$ All identifiers that begin with a letter and contain only letters, digits, and underscores,

but cannot end with an underscore.