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\begin{array}{ll}a_{n}=a_{1}+(n-1) d & \\a_{4}=a_{1}+3 d & \\a_{4}=8+3 d & \text { Write the four th term of the sequence in terms of } a_{1} \text { and } d . \\14=8+3 d & \text { Substitute } 14 \text { for } a_{4} . \\d=2 & \text { Solve for the common difference. }\end{array}