3r-6=-21
3r-6+6=-21+6 (Add 6 to both sides...)
3r=-15
r=-5
3r-6=21
3r-6+6=21+6 (Add 6 to both sides...)
3r=27
r=9
r=-5 or r=9
Since the deepest point of Death Valley is below sea level, its altitude is negative. So, the difference between the altitudes is

Subtracting a negative means to add, so we have

There are 3 terms in this expression. You can determine this answer by knowing that terms are either a single number or variable or numbers and variables multiplied together. Here's a picture example below:
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The midsegment of a trapezoid is the segment connecting the midpoints of the two non-parallel sides. In trapezoid below, segment P Q is the midsegment. The length of the midsegment of trapezoid is half the sum of the lengths of the two parallel sides. In the figure above: P Q = A B + C D 2.