consider the base to be 21 cm and the other two sides to be 17 cm and 10 cm. now draw an altitude of z cm. the altitude divides the base into x and y cm. Using Pythagoras theorem, x^2 +z^2=289.(1) y^2+z^2=100.(2) subtracting 2 from 1 we get x^2-y^2=189. (x+y)(x-y)=189. we know (x+y)=21, therefore (x-y)=9. from that we get x=15 and y=6. with that we find that z=8. therefore area= 1/2 * 8* 21=84 cm squared. I feel this is easier than heron's formula. with some practice, this will become a very effective and fast method for finding area of any triangle with only it's sides given.