How do i perform this operation(-2.1) + (3.26)

You are adding a negative and a positive.  It is like subtracting 2.1 from 3.26.  Because 2.1 is negative, whenever you add it to a number, you actually subtract.  To do this:
3.26 + (-2.1) is like 3.26 - 2.1
3.26 - 2.1 = 1.16