\hspace{n}

inserts a space of n pixels
Ex.: $$$f(x)\hspace{6}=\hspace{6}0$$$ gives $$f(x)\hspace{6}=\hspace{6}0$$
can be combined with the preceding command \unitlength{m}(default: m=1px) , which defines the applied unit
Ex.: $$$\unitlength{20}a\hspace{2}b$$$ gives $$\unitlength{20}a\hspace{2}b$$ , i.e. a space of 20x2=40px