Hey there!
Let's do this step by step:
1. P-13=2 -------Write out the equation
2. P-13+13=-2+13 ------- Undo the operations
3.

=

------- What you get
4. P=11
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Answer:
x=-1
Step-by-step explanation:
f (x)= -x + 8
Let f(x) = 9
9 = -x+8
Subtract 8 from each side
9-8 = -x+8-8
1 = -x
Multiply each side by -1
-1 = x