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If someone takes away a five-dollar debt from you, you just got five dollars richer. In algebra, subtracting a negative quantity works the exact same way.
Distributing a negative sign across parentheses flips the sign of every single term trapped inside.

The Invisible Negative One

When a minus sign sits directly outside parentheses with no number attached, like −(3x−4)-(3x - 4), think of it as multiplying everything by −1-1.
Because multiplying by a negative number reverses a sign, positive terms become negative, and negative terms turn positive.
So, −(3x−4)-(3x - 4) expands into (−1)(3x)+(−1)(−4)(-1)(3x) + (-1)(-4), which simplifies neatly to −3x+4-3x + 4.
What happens when the number outside isn't just a minus sign, but a larger negative value?

Distributing Negative Factors

A coefficient is a numerical value multiplied by a variable, such as −4-4 in the expression −4(2x−5)-4(2x - 5).
To expand this expression, multiply the entire factor of −4-4 by each term inside the parentheses separately.
The first step gives (−4)(2x)=−8x(-4)(2x) = -8x, and the second gives (−4)(−5)=+20(-4)(-5) = +20.