As pointed out by S.McGrew, the mica flake increases the path length for light passing through it owing to the fact that light travels slower in mica as compared to vacuum.
It is already mentioned that the mica flake is 'thin', so we can can make some reasonable approximations to arrive at the answer. Also, as nothing has been specifically mentioned, we can assume that there is no loss of energy as the light passes through the flake. So no, the pattern would not get darker, its intensity remains the same.
Yes, the whole pattern would be shifted upwards because if the added path length which the light has to travel. Now you for solving the question, you need to get an expression for the extra path length. This can be done with the concept of 'optical path', the distance which light would have traveled in the same time, had it been in vacuum. You can derive it to be the geometrical path times the refractive index of the medium.(Try deriving it.) If the refractive index is n, the extra path length would be $= nt-t$ , where t is the thickness. Now you add thid term to the expression for path difference and equate it too 7λ and solve the equation to get the value of t.
P.S. : Keep in mind that we have assumed the mica flake to be 'thin'.