Studs x6 is a Red Brick in LEGO City Undercover which multiplies the value of collected studs by six. This stacks with other stud multiplier bricks for a maximum multiplier of 3840.
During Special Assignment - High Steal. After getting through the first gate, to the left are some shacks with fire doors. The red brick is in the first from the right. Use the Fireman's axe to open it.