## Elementary Algebra

First, we simplify the expression: $-3a-1+7a-2$ =$-3a+7a-1-2$ =$4a-3$ We evaluate the expression by substituting the value of $a=0.9$ into the expression: $4a-3$ =$4(0.9)-3$ =$4(\frac{9}{10})-3$ =$\frac{36}{10}-3$ =$\frac{36-3(10)}{10}$ =$\frac{6}{10}$ =$0.6$