Two sides of a triangle measures 25 cm + 35 cm which of the following would be the measure of the third side

Assuming those are the legs of the triangle and that you need to find the hypotenuse... I would use the formula: a^2+b^2=c^2
It doesn't matter what a and b are, just fill it in.
25^2+35^2=c^2
625+1225=c^2
1850=c^2
Take the square root of 1850, it isn't a perfect square so I got: 43.01.
If you want, just round it to 43, idc. 
So, if I were to check my work, I would need to remember that the hypotenuse is the longest side.
Since 43 is the hypotenuse, it is the longest side.