# How do you factor 25x^2 + 49?

If we are working with polynomials over the complex numbers, then this is a difference of squares and can be factored using $- 49 = {\left(7 i\right)}^{2}$, so
$25 {x}^{2} + 49 = 25 {x}^{2} - \left(- 49\right)$
$= \left(5 x + 7 i\right) \left(5 x - 7 i\right)$