C++ :: Get Angle Of Bullet After Bouncing?

I'm trying to make a bullet bounce after it hits a wall. I think bouncing on the top and bottom wall works perfectly, but it won't bounce off of the left and right walls. How would I get it to bounce? This is how I get the direction the bullet it going whenever I shoot.

player.dir = GetAngle(player.mouseX, player.mouseY, player.x, player.y);
float GetAngle(float x1, float y1, float x2, float y2) {
return atan2(y1 - y2, x1 - x2);
}

Also what I was trying to do to make the bullet bounce.

if (bullets[i].x < 10 || bullets[i].x > screenWidth - 10 ||
bullets[i].y < 10 || bullets[i].y > screenHeight - 10) {
if (bullets[i].type == 8)
bullets[i].dir *= -1;
}