Your function is not a function of and ...
"Alignment"is not the problem. In both of your posts, you have said "if (x, y) is not 0" but have written a and b in the formula. Do you mean
"[tex]f(x, y)= x^4+10x^2y^2+21y^4[tex] if (x, y) is not (0, 0), f(0, 0)= t"?
This function is clearly continuous everywhere except possibly at (0, 0). A function is continuous at a given point if and only if the value of the function there is equal to the limit of the function as (x, y) goes to that point. This function will be continuous at (0, 0) if and only if
What is that limit?