The answer is a -16.
How to solve an equation in one variable?
When you only have an equation in one variable to solve you will transpose the variable together and get the outcome on the other side.
How to transpose?
When you transpose a variable or a constant on the other side, their function gets changed, so you can add, subtract, multiply or divide on both the sides the effect would not change, but you have to keep in mind about the equality holding true.
Solving the given problem
9d+1=8d-15
As you have to group the variable then subtract 8d on both sides
d+1=-15
After that, you transpose +1 from LHS to RHS
d = -16
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