First, let's simplify the expression within the parentheses:

1/3 - 5/6 = 2/6 - 5/6 = -3/6 = -1/2

Next, let's simplify the expression within the brackets:

1/5 * 3/4 = 3/20

Now, let's substitute these values back into the original expression:

-1/2 : 17/26 + 1/2 * 3/20

-1/2 : 17/26 + 3/40

-1/2 : (17/26 * 20/20) + 3/40

-1/2 : 340/520 + 3/40

-1/2 : 340/520 + 39/520

-1/2 : 379/520

To divide fractions, we multiply by the reciprocal of the second fraction:

-1/2 * 520/379 = -520/758

Therefore, the final answer is -520/758 or approximately -0.6855.