Take away the parentheses: 3.4a-1.7b+2.5a-3.9b.
Collect like terms together: 3.4a+2.5a-1.7b-3.9b.
Add the coefficients for each variable: 3.4+2.5=5.9 and -1.7-3.9=-5.6 and join the new coefficients to their variables:
5.9a-5.6b
Note that the variables are still separate: you can't join two unknowns.