To simplify this expression, first convert the mixed number into an improper fraction:

(-7/12 + 5/6):(-1 5/6)
= (-7/12 + 5/6):(-11/6)

Next, divide the numerator by the denominator:

= [(-7/12 + 5/6) * (-6/11)]

Now simplify the expression within the brackets:

(-7/12 + 5/6):(-11/6)
= [-42/72 + 60/72]:(-11/6)
= (18/72):(-11/6)
= (1/4) * (-6/11)
= -6/44
= -3/22

Therefore, (-7/12 + 5/6):(-1 5/6) simplifies to -3/22.