Three consonants run one idea in two directions: bringing things to a total. Add up numbers and you reach a sum; add up a discussion and you reach a summary; bring two minds to the same total and you reach agreement. The same root that closes an arithmetic line closes a negotiation. Every word here is verified against pealim, niqqud included.
The same root runs through the verb system. Each binyan is a voice: it bends the one idea into active, passive, causative or reciprocal. Tap to hear.
Watch the split: the everyday verb for summarizing is the pi'el, while agreeing rides the hif'il ("to bring oneself to the same total"). The pa'al survives mostly in formal or literary use. The passives nif'al, pu'al, huf'al and the reflexive hitpa'el are not lexicalized here.
Same root letters, different noun moulds (mishkalim). The mould tells you the function: who does it, the act of doing it, or the thing produced.
Strip the prefixes, suffixes and vowels. Three consonants remain, and the meaning lives there.