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Answer:
a = (s +t)/(m +r)
Step-by-step explanation:
Add t, then divide by the coefficient of 'a'.
[tex]a(m+r)-t=s \qquad\text{given}\\\\a(m+r)=s+t \qquad\text{add $t$}\\\\\boxed{a=\dfrac{s+t}{m+r}} \qquad\text{divide by $m+r$}[/tex]
