To begin factoring, set the equation equal to zero - subtract 10x from each side:
5x² = 10x
5x² - 10x = 0
Next, see if you can pull out a GCF, --greatest common factor-- from both terms. You can pull out 5x:
5x² - 10x = 0
5x(x - 2) = 0
The most you can factor the expression is 5x(x - 2) = 0.