So to find the answer you want to isolate the y. So the first thing you want to do is to move everything to the opposite side of the = sign. You start by adding 10 to both sides.
10+ 5y -10 = -25 +10
5y = -15.
the tens cancel out on the left side and on the right you get left with -15/
Since y is being multiplied by 5 you always want to do the opposite so you divide by 5.
5y/5 = -15/5
y = -3
It's hard to explain but you just have to remember that what you do to one side you must do to the other to keep the equation balanced.
The cost of meal = 
The tip to the waiter =
%
Discount =
%
Cost = 
Cost = 
Cost = $
0
Find the x and y intercepts or any other 2 points. (I advise you finding the y intercept though.)
then calculate for slope using:
y(2) - y(1)
________
x(2) - x(1)
The (2) and (1) are not exponents they are small numbers at the bottom of the numbers.
The slope is: 4
Use the Pythagorean therm , you know the a^2+b^2=c^2