There are other ways that we could figure it [the number of casualties in a disaster] out--counting death certificates, or crime reports. It's a multivariable equation; as long as you have enough variables, you can solve for the missing ones, and check your answers.A multivariable equation can not be solved. You can solve a system of multivariable equations, but only if you have at least as many equations as you have variables.
She means to say either:
1. There are many pieces of data. As long as you have all these values, you can plug them into an equation and find out the number of casualties.
2. There are many equations, each of which provides a different estimate of the number of casualties. You can do them all, and then check how each method compares.
I'm not sure if she means no.1 or no. 2. Because being precise in stating mathematical things (even in an article on arithmetic!) is not a concern of journalists, there is no real way to tell.