Can someone explain me where this 13 came from, ? (sorry my poor english i'm brazilian)
a-7=3(b+2)
a=3(b+2)+7
a=3b+13

Hey there,

a-7=3(b+2), we do 3 times b and 3 times 2 according to the rule.

a-7=3b+6 we change + to - and - to + when move nums from 1 side to another one.

a=3b+13 we get the solution.

Hope this helps :)

0.02

step-by-step explanation:

time of 9 192 631 770 periods of the radiation corresponding to the transition between the 2 hyperfine levels of the ground state of the caesium 133 atom at 0 k (but other seconds are sometimes used in astronomy)