If a triangle has 3 side one is 12 one is 72 what is the last sude

this triangle has 3 sides one is 12, lets call this side a²

another side is 72, lets call this side c²

this triangle must follow Pythagoras therom if it is right angled, that is a²+b²=c²

where a and b are the two shorter sides and c is the longest side (called the hypotenuse) opposite the right angle

to find the last side you do 72² - 12² = 5040

but it is the square root of 5040 which is 70.99 (two decimal places) if you want it in decimal form, if you want it in surd form it is √5040

There is no way to answer that with only the information given.  
The 3rd side can be ANY length between 60 and 84.  You can’t
solve a triangle if you only know 2 sides of it.

And even if it possibly is, maybe perhaps, how did you decide that 72 is the longest side. Couldn’t these two numbers be the two legs? Too much info missing for a unique solution.