If you study mathematics or like math in general, the math related katas are fun.
If you are searching for a job or you are about to take a test, you are more stressed to learn and you're interested in the "useful" ones, so math related katas can be annoying in that case.
I had similar issues in the past. The thing that helped me was filtering katas, on the left part of the screen, select the appropriate tags.

Agree, should be at least 7.

The description for Javascript:
Return n if the input is already a multiple of three, and return -1 if no such number exists.

Should be:
...and return null if no such number exists.

Else you don't pass the first 2 tests.

x(x+1) = n =>
x**2 + x - n = 0

So you need to solve this quadratic equation: x**2 + x - n = 0
The solutions(or roots) are: x1,2 = [-1 +- sqrt(1+4*n)] / 2

This has solutions if sqrt(1+4*n) is integer.

Tell me if you need more clarifications.

! is the NOT operator, so !zombies, read it like "no zombies". It is the same condition as zombies == 0.

if (!zombies): if zombies == 0 then zombies evaluates to false and then !false = true
if zombies != 0 then zombies evaluates to true and then !true = false

I hope it helped you.