assumptions have to be made because the question is poorly-worded

z varies directly with the sum of the squares of x and y implies the following formula, where 'k' is the constant of variation:

z = k(x^2 + y^2)

to solve for 'k', simply substitute in the given numeric values: z=5 when x=2 and y=6

if you are not wanting to solve for 'k', then the problem statement simply needs to be more complete