please do ALL 3 will give brainliest!! and a surprise!!!

Answer:
a) -2.5x+3.9+1.7x
circle: 2.5x and 1.7x
variables: x
b) 1 1/2+t+2/3s
no like terms
variables: s and t
c) 15k+2/3k+3m
circle: 15k and 2/3k
variables: k and m