If f(x) = x2 − 1, and f(2a) = 35, then what could be the value of a

What's the difference between: x≥0 and nonnegative integer. ?

Which of the following is a function? a.) {(-2, -4/5), (-1,-,,-1)} b.) {(-2,,1/,3/,1)} c.) {(-22)} d.) {(-21)}

With finding limits of multi variable !