Function estimated with differential

I just have a basic question but it is making it impossible to finish several problems.

When you use the differential of a function(x,y), the partial derivative with respect to x times change in x + partial derivative with respect to y times change in y to estimate the value of a function,

do you add both of these terms to the original function or do you subtract them??? I think from looking at formula used for a tangent line approximation, you would add them but my book does not cover this and I am not sure. Thanks again for answering such a basic question. Frostking