A red laser ( lambda r = 6 3 3 nm ) is shone through a

 A red laser $($lambda λ _r$=633$ nm$)$ is shone through a narrow slit located $2$ m from a screen. When the light source is replaced by a blue laser $(\lambda$ _b $=488$ nm$)$ the position of the first minimum of the intensity pattern on the screen moves by $3$ mm$.$ Calculate the slit width.