A linear function of x is a function of the form y = ax + b, where a
and b are constants. It's called a "linear" function because a
plot of the x and y coordinates for which this equation is true takes the shape
of a straight line.
More about the pages of information about linear functions and linear
equations.
Patterns -- given a table of x and y
values that are linearly related, how can you spot the formula that relates
them?
Linear Regression -- how to calculate
the linear regression as a descriptive statistic