To simplify this expression, we will first convert the trigonometric functions into their corresponding ratios:

cos(-45°) = cos(45°) = √2/2
sin(0°) = 0
tg(30°) = sin(30°) / cos(30°) = (1/2) / (√3/2) = 1/√3

Now we can substitute these values back into the expression:

2cos(-45°) + 5/7sin(0°) - 3tg(30°)
= 2(√2/2) + 5/70 - 3(1/√3)
= √2 + 0 - 3/√3
= √2 - 3/√3.

Therefore, the simplified expression is √2 - 3/√3.