Let us discuss . . . . . . . . matter seriously.

Articles are used to determine whether a noun is definite or indefinite in a sentence. 'The' is used when we want to specify something definite (a particular person or thing). On the other hand, 'a' and 'an' are used when the noun we want to refer is not something specific but a general one.
Here, we want to focus on one particular thing i.e. "matter". We are saying something specific that needs to be discussed seriously. So, 'the' article will be used.