Question

2p + 3x = 680 x = 2p

Answer 1

P=85

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Answer 2

Use substitution. 

2p + 3(2p) = 680
2p + 6p = 680
8p = 680
p = 85

Plug in.
x = 2p
x = 2(85)
x = 170
p = 85