Chapter 11 - Quadratic Functions and Equations - 11.1 Quadratic Equations - 11.1 Exercise Set - Page 706: 18

$t=\pm 2i$

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}