Answer: 60 degrees
Explanation: If all angles are same, each angle is 60 degrees and adds up to 180.
Take the homogeneous part and find the roots to the characteristic equation:

This means the characteristic solution is

.
Since the characteristic solution already contains both functions on the RHS of the ODE, you could try finding a solution via the method of undetermined coefficients of the form

. Finding the second derivative involves quite a few applications of the product rule, so I'll resort to a different method via variation of parameters.
With

and

, you're looking for a particular solution of the form

. The functions

satisfy


where

is the Wronskian determinant of the two characteristic solutions.

So you have




So you end up with a solution

but since

is already accounted for in the characteristic solution, the particular solution is then

so that the general solution is
Midpoint formula
mid of (x1,y1) and (x2,y2) is

so
(22,15)=

therefor
22=

and
15=

slv each
22=

times both sides by 2
44=18+x1
minus 18 both sides
26=x1
15=

times both sides by 2
30=6+y1
minus 6
24=y1
the other point is (26,24)
2/3+1/3x=2x
2+1x=6x
x-6x=-2
-5x=-2
x=2/5