To combine like terms, add or subtract the coefficients (the numbers that come before the variables) of terms that have the same variable raised to the same exponent.

2a + 7a = 9a

The slope-intercept equation for a line is y = mx + b where m is the slope and b is the y-intercept of the line. From the graph, you can see that the y-intercept (the y-value from the point where the line crosses the y-axis) is -3. The slope of this line is the change in y divided by the change in x. The endpoints of this line segment are at (-2, 1) and (2, -7) so the slope becomes:

Plugging these values into the slope-intercept equation:

y = -2x - 3

A right angle measures 90 degrees and is the intersection of two perpendicular lines. In diagrams, a right angle is indicated by a small box completing a square with the perpendicular lines.

The entire length of this line is represented by AD which is AB + BD:

AD = AB + BD

The surface area of a cube is (2 x length x width) + (2 x width x height) + (2 x length x height):

sa = 2lw + 2wh + 2lh
sa = (2 x 9 x 9) + (2 x 9 x 7) + (2 x 9 x 7)
sa = (162) + (126) + (126)
sa = 414