What is the gcf of 5x plus 35?

"GCF"s are usually only relevant to numbers, not to algebraic expressions.

But even if they applied to expressions, the "C" in GCF stands for "common", which

means "same for both", and you only listed a single excpression, so there can't be

anything 'common' about it.

In short, your question loses in at least two ways.