I don't understand when to use the formule with the constant k, and when to use the formula without it.

In the exercise that i sent here, they use the formule with a constant but I don't get why.

Can someone help me out please?
Thanks.

I dont understand when to use the formule with the constant k and when to use the formula without it In the exercise that i sent here they use the formule with class=
I dont understand when to use the formule with the constant k and when to use the formula without it In the exercise that i sent here they use the formule with class=