15 - Given a system of two equations, we can produce an equivalent system by replacing one equation by the sum of the two equations, or by replacing an equation by a multiple of itself.
16 - It tells us that two expressions mean the same thing, or represent the same number. An equation can contain variables and constants.
The linear equation can be written in the form y = mx + b. "y" and "x" stay as they are. The "m" is the slope or rate of change. The b is the y-intercept.
