$6.35 \times 6.35 + 2 \times 6.35 \times 3.65 + 3.65 \times 3.65 = ?$

This is the form $a^2+2ab+b^2=(a+b)^2$ with $a=6.35,\ b=3.65$. Therefore it equals $(6.35+3.65)^2=10^2=100$.