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:S If the discriminant is positive it's indefinite.The coefficient of x2 is already +ve so you just need the discriminant to be +ve.
So...
b2 - 4ac > 0
36 - 36m2 > 0
-36m2 > -36
m2 < 1
m2 - 1 < 0
(m-1)(m+1) < 0
-1 < m < 1