Answer: -2t + 220
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Step-by-step explanation:
For now I'll use x in place of t.
Two points on that line segment are: (15,190) and (25,170)
Use the slope formula to get the following

The slope is -2.
A slope of -2 means we go down 2 each time we move to the right 1.
In short: "down 2, right 1".
Now use point-slope form to determine the equation

The last step to do is replace x with t and we get y = -2t+220 which is the final answer.
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Check:
Plugging in t = 15 should lead to y = 190.
y = -2t + 220
y = -2(15) + 220
y = -30 + 220
y = 190
So far so good.
Now we need to check if t = 25 leads to y = 170.
y = -2t + 220
y = -2(25) + 220
y = -50 + 220
y = 170
That works as well.
We have confirmed the points (15,190) and (25,170) are on this equation. The answer is confirmed.