#### Answer

$t=\pm 2i$

#### Work Step by Step

Since $x^2=a$ implies $x=\pm\sqrt{a}$ (the Square Root Principle), the solutions to the given equation, $
t^2+4=0
,$ are
\begin{array}{l}\require{cancel}
t^2=-4
\\\\
t=\pm\sqrt{-4}
\\\\
t=\pm\sqrt{-1}\cdot\sqrt{4}
\\\\
t=\pm i\cdot2
\\\\
t=\pm 2i
.\end{array}