So slope intercept form is y=mx+b m is the slope and b is the y intercept. The equation of this line would be y=1/4x+2 because the rise over run (slope) is 2/8 or 1/4 and the line intercepts the y axis at 2.
Try this suggested solution, note, 'D' means the region bounded by the triangle according to the condition. It consists of 6 steps.
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4 + 1.3 = 5.3
5.3 - 5.7 = -.4
The temperature at 10pm was -.4°F
Correct me if I'm wrong. :)