I made a picture to demostrate what I need,basecaly I have a bullet with velocity and I want it to bounce with the correct angle after it hits a circle

Solved(look the accepted answer for explain):

Vector.vector.set(bullet.vel);    //->v
Vector.vector2.setDirection(pos, bullet.pos);     //->n normal from center of circle to bullet
float dot=Vector.vector.dot(Vector.vector2);    //->dot product
Vector.vector2.mul(dot).mul(2);
Vector.vector.sub(Vector.vector2);  
Vector.vector.y=-Vector.vector.y;  //->for some reason i had to invert the y
bullet.vel.set(Vector.vector);