You take the inverse of the total resistances of each branch and add them up.
So if you have 5ohm, 7 ohm, and 10ohm, you would add
1/5 + 1/7 + 1/10 = 31/70
Then flip it back by either using the x−1 (inverse) key on your calculator or simply dividing 70 by 31 to get a total of 2.26ohms
