The equation is 

               (Subtract 6 from each side)

                          (Apply Additive inverse property: 6 6= 0)

                              (Subtract:    )

Check:

To check the solution, substitute  in original equation.

                               (Add: - 15 + 6 = - 9)

The statement is true.

 Graph the solution on number line.

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• First draw a number line.
\
• Mark a dot at -15 on  the number line.
\

The value of t = - 15.