The factors of 50a³ are 1, 2, 5, 10, 25, 50,
and their products with a, a² and a³ .
The factors of 10a² are 1, 2, 5, 10,
and their products with 'a' and a² .
Their common factors are 1, 2, 5, 10,
and their products with 'a' and a².
Their greatest common factor is 10a² .
(Another way to spot it, easily, is to remember this helpful factoid:
If the smaller number is a factor of the larger number,
then the smaller number is their greatest common factor.
Using the greatest common factor, then . . .
50a³ + 10a² = 10a²(5a + 1) .