The answer is = (x + 5y) (x + 7y)
Break the expression into two groups.
x^2 + 12xy + 35y^2
(x^2 + 5xy) (7xy + 35^2)
Factor out x from x^2 + 5xy: x(x + 5y)
Factor out 7y from 7xy + 35y^2: 7y(x + 5y)
=x(x + 5y) + 7y(x + 5y)
Next, factor out the common term (x+ 5y).
Answer = (x + 5y) (x + 7y)