Question 535670
{{{(-5y+4x)-(-3x+y)=-5y+4x+3x-y=7x-6y}}}
The first parenthesis was optional, since there were no operations done on it other than addition and subtraction that would be done last anyway.
The second parenthesis directly follows a minus sign, and to remove it and still get an equivalent expression you need to flip all the plus to minus signs and vice versa.
Want to know why?
Suppose you have $8.00 in your pocket and want to buy a meal costing $4.95. You reason that {{{4.95=5.00-0.05}}}
You give the cashier $5.00, get $0.05 in return, and are left with $8.05.
What happened here can be described as
{{{8.00-4.95=8.00-(5.00-0.05)=8.00-5.00+0.05}}}
To subtract (5.00 minus 0.05), you subtracted 5.00 and added 0.05.
You eliminated the parenthesis and the minus sign in front of it too. You changed the plus sign between the parenthesis and 5.00 (it's invisible) to a minus sign, and the minus sign in front of the 0.05 in the parenthesis to a plus sign.