The LCM of 24 and 72 is 72 :)
My answer was wrong, delete this answer or disregard
The characteristic solution follows from solving the characteristic equation,

so that

A guess for the particular solution may be

, but this is already contained within the characteristic solution. We require a set of linearly independent solutions, so we can look to

which has second derivative

Substituting into the ODE, you have



Therefore the particular solution is

Note that you could have made a more precise guess of

but, of course, any solution of the form

is already accounted for within

.
Answer:
When you have two negative signs, one turns over, and they add together to make a positive. If you have a positive and a negative, there is one dash left over, and the answer is negative.
Step-by-step explanation:
It’s 2 because if you finish the equation