Why do we call facts facts? We all observe them to be proven true time and time again.

A consensus is an agreement to believe in the same thing (according to the mirriam-webster dictionary). So, looking at that as a basis, we can conclude that our choice in numeral-base system is an opinion. However, this does not prove math to be an opinion.

In decimal, we count as 1, 2, 3, 4, 5...ad infinitum

These numerals (both written and oral) are merely a representation of a concept. We count objects in this manner, and will always get the same result regardless of the bas-system chosen (that is to say, just because the chosen representation sounds and/or looks differnt, it doesn't change the actual ammount of the objects we are counting).

So, keeping that point in mind, we can now say math is, indeed, fact and not opinion. In decimal, when we have 2 of an object added with 2 more of an object, we will always end up with 4 of that object. In binary, we represent this as 10 + 10 = 100. The representation is different, but the actual action is the same.

If we use the basis of math as an opinion, then I could say 1 + 50 = 5 in decimal. This is impossible, as the physical objects being represented by these figures do not work in this manner.

And that's only going over the most basic mathematical systems. We have calculus, triganometry, laplace transforms (my personal least favorite -.-), matrix math...there are so many equations all because they have been shown to always* be true.

(* always is to be taken loosly, as there are cases where an equation is wrong for a set of examples, which leads to changes to the equation to explain the variance in results (such as in the many principles, theorems, and laws I had to learn to earn my degree, heh))