Question 5523
There are no following numbers in this equation... did you forget to type something?

the common factors are discovered by factoring all terms into prime factors and then multiplying the common terms together:
{{{40s^5t^3u^2}}} = 2*2*2*5*s*s*s*s*s*t*t*t*u*u
{{{40s^3tu^4}}} = 2*2*2*5*s*s*s*t*u*u*u*u

Both terms have 2*2*2*5*s*s*s*t*u*u
so the answer is 
{{{40s^3tu^2}}}