You can combine terms if they have the same variables and powers
12f - 3f = (12-3)f = 9f
7.5g + 3.5g = (7.5+3.5)g = 11g
180h + 35j h and j are different variables so these terms cannot be combined
(3/4)k + (1/4)k = (4/4)k = k
7m + m = (7+1)m = 8m
5n + n = (5+1)n = 6n
You don't have to include the middle step where I used the distributive law to factor out the variable, I just wanted to include that extra step so you could see what's going on.