Here's two solutions:
In[3]:= NSolve[{((2*a - 2*b + 6 b*c + 2*d)^2 (1 -
k)^2*3.5^2*(2*a + 2*b +
2*b*c)^2/(f^2*(1 - a - 2*b*c - d)^2*(2 - a - 2*b)^2)) ==
1.13*10^-3, ((2*
b*(1 - c)*(2*a + 2*b + 2*b*c))/((2 - a - 2*b)*(2*a - 2*b +
3*2*b*c + 2*d)*(1 - k))) ==
0.236, (((2*a - 2*b + 3*2*b*c + 2*d)*(1 - k))^3*(2*a + 2*b +
2*b*c))/(f^2*(1 - a - 2*b*c - d)*(2*b*(1 - c))) ==
2.15*10^-3, ((2*a - 2*b + 6*b*c + 2*d)^2*3.5*(1 - k)^2)/(f*(1 -
a - 2*b*c - d)) ==
0.154, (2*a - 2*b + 6*b*c + 2*d)*(1 - k)/f == 0.01/3.5,
1 + 2 + 4*b*c - 2*a*k + 2*b*k + 6*b*c*k - 2*d*k + 2*a - f == 0}, {a,
b, c, d, k, f}, Reals, Method -> {"UseSlicingHyperplanes" -> False}]
Out[3]= {{a -> 120.931, b -> -55.5727, c -> 1.00973, d -> -6.99815,
k -> 5.66587, f -> -3807.97}, {a -> 170.639, b -> -90.2124,
c -> 0.994008, d -> 9.23793, k -> -2.34647, f -> 2515.58}}