$211^{870}$ has unit digit $1$ and unit digit of $146^{127}$ is $6$ and unit digit of $3^{424}$ is $1$

because $424$ repeats remainder pattern $3,9,7,1$ so on remainder divisible by $4$ cases unit digit is $1.$

So in multiplication part multiplication unit digit = $6$ as $6*1$ which is added to $1$ hence answer is $7.$