That depends on what variable you want to take the derivative with respect to. With respect to #x# the answer is #7*y#, while with respect to #y# the answer is #7*x#.

When you take a derivative of a function you get an expression that represents the rate of change or slope of that function. With single variable calculus you usually only ever take derivatives for a function with respect to the independent variable. This is usually represented by #x#. However, with multivariate calculus you often times have a function defined in three or more dimensions. For instance the above function might look something like this.

#z=7*x*y#

You could then choose to take the derivative with respect to either the #x# axis or the #y# axis. If you take the derivative with respect to the #y# axis you will get an expression representing the rate of change or slope in the #y# direction, while if you were instead take the derivative with respect to the #x# axis you would get an expression representing the slope in the #x# direction. The process to take that derivative is as simple as treating the other variable as if it were a constant like the #7#.

#(dz)/(dy)=7*x#

#(dz)/(dx)=7*y#