M is the slope of the equation.
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If your looking for a simplified answer, it would be

.
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Answer:
d = 7
Step-by-step explanation:
The nth term of an arithmetic sequence is
= a₁ + (n - 1)d
where a₁ is the first term and d the common difference
Give a₁₀ = 67 and a₁ = 4 , then
a₁ + 9d = 67
4 + 9d = 67 ( subtract 4 from both sides )
9d = 63 ( divide both sides by 9 )
d = 7
he would have traveled 210 miles because in half an hour he would have traveled 30 miles and in 3 hours he would have traveled 180 miles. so we have to add those up and we get our answer: 210 miles