Question

find the zeroes of the quadratic polynomial
t square-15

Answer 1

You haven't written RHS value of the equation. I think you forgotten to write 0 in RHS.
Assuming that RHS is 0
we have the equation:-
$t^{2}-15=0$
$t^{2} =15$
$t = \sqrt{15}$ or $t = -\sqrt{15}$
So the roots or zeros are - $\sqrt{15}$ and $-\sqrt{15}$

Answer 2

t² - 15 = 0
⇒ t² = 15
⇒ t = √15 and -√15