You are solving for the value of the variable, x. Note the equal sign, what you do to one side, you do to the other.
First, simplify by combining like terms:
-8x - x + 7 = 25
(-8x - x) + 7 = 25
-9x + 7 = 25
Isolate the variable x. Note the equal sign, what you do to one side, you do to the other. Do the opposite of PEMDAS. First, subtract 7 from both sides:
-9x + 7 (-7) = 25 (-7)
-9x = 25 - 7
-9x = 18
Isolate the variable, x. Divide -9 from both sides. Note that when you are dividing by a negative number, your answer will have the opposite sign of the sign given in the equation:
(-9x)/-9 = (18)/-9
x = 18/(-9)
x = -2
x = -2 is your answer.
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