[R] how to make partial mean() of a matrix only when second value matching some logic

[madr]

for example I have matrix with two columns

x,y
1,0.56
2,9.55
2,7.56
5,2.55
5,0.56
3,0.55
2,0.56
2,1.56

so I need to take average from y values placed where x==2
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