The x-axis is the horizontal axis on a graph, and the y-axis is the vertical axis. The x-intercept is the point a line, represented by a function, where it crosses the x-axis on the graph. The x-intercept is written as (x, 0), because the y-coordinate is always zero at the x-intercept. If you know the slope and the y-intercept of the function, you can calculate the x-intercept using the formula (y - b) / m = x, where m equals the slope, y equals zero, and b equals the y-intercept.

Substitute the known slope for m and the y-intercept for x in the equation (y - b) / m = x. For example, if the slope equals 5 and the y-intercept equals 3, write the formula as (y - 3) / 5 = x.

Substitute 0 for y in the equation, since the value of y is zero In this example at the x-intercept. Using the previous example, (y - 3) / 5 = x, the equation becomes (0 - 3) / 5 = x.

Solve the equation for the value of x. Using the previous example, (0 - 3) / 5 = x, solve the numerator first. Subtract 0 from 3 to get negative three. The result is -3 / 5 = x. Convert the fraction to a decimal by dividing -3 by 5, and the result is -0.6. The x-intercept equals -0.6.