Arithmetic Sequence: original formula: Sn=n/2 (2u1+(n-1)d)

problem formula: S20= 20/2 (2x60 (20-1)(-3)) =630.

The sum of the sequence is 630, but, i need help understanding HOW it is 630. i dont know what to multiply/add/divide in the series first, i don't understand how they got the answer to be 630. Could someone give a detailed explanation on how to work out the problem and parentheses step by step? Thank you