Where's my value?

Pyth, 2 bytes

0-indexed.

xE

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Explanation

xEQ  - Full Program. Takes Input from standard input. Q means evaluated input and is implicit at the end of the program.

x   - Get all the indexes of x in y
 E  - Evaluated Input #2 - The value
  Q - The list - Evaluated Input #1

Python 3, 45 bytes

-3 bytes thanks to @EriktheOutgolfer and @Chris_Rands

lambda y,x:[i for i,j in enumerate(x)if j==y]

Test Suite.

Today I learned enumerate(x) == zip(range(len(x)),x).

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Python 3, 47 bytes

lambda n,l:[x for x in range(len(l))if l[x]==n]

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MATL, 2 bytes

mf

The m consumes two arguments, and checks each element in the array whether is equal to the other argument, f returns the indices of the truthy entries of an array.

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