The question is ambiguous. Is it "d-6c equals 17"? Or "d<(6c=17)"? Or "d-6c<17"? Or "d-6c<17? Or "d<6c=17"?
Whichever, there are two unknowns, c and d, and only one equation or inequality, with the exception of the second interpretation, where 6c=17 so c=17/6=2.83 and d<2.83. The last interpretation is the question asked and makes no sense, having both an equality and an equality. The other interpretations give a relationship between c and d: d=17+6c; d<17+6c; d<17+6c.