Use the Pythagorean Theorem OR the distance formula (they are based upon the same principles).
The x-component of this distance is the distance from -7 to +9, which is 16. If you're unsure of why, graph a number line, plot -7 and +9 and then count the number of scale divisions between the two.
The y-comp is the distance from 2 to 2, which is zero.
Thus, the distance between the two points is sqrt(16^2+0^2), or 16.