12.9 = x + 7.1
subtract 7.1 from both sides to isolate the variable, x.
5.8 = x
5.8 is your answer.
and 
There are 6 possible outcomes when rolling a die, 1, 2, 3, 4 , 5, 6
A
numbers less than 4 are 1, 2, 3
P( number less than 4 ) =
= 
B
numbers greater than 4 are 5, 6
P( number greater than 4 ) =
= 