Finding the Order of SL(2,Fp) in Abstract Algebra

Fulger
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Help with abstract algebra!

Here is my quetion.
What is the order of the set of all 2x2 matricies (such that its entries a,b,c,d are between 0 and p-1), and whose determinant is
congruent to 1 modulo p ?
=> Order of SL(2,Fp)
thanks :)
 
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