1) -x + 3.9 = 2.1

To solve for x, first subtract 3.9 from both sides:

-x = 2.1 - 3.9
-x = -1.8

Next, divide both sides by -1:

x = 1.8

Therefore, the solution is x = 1.8

2) -x + (-5.16) = 6.06

To solve for x, first add 5.16 to both sides:

-x = 6.06 + 5.16
-x = 11.22

Next, divide both sides by -1:

x = -11.22

Therefore, the solution is x = -11.22