Algebra

Cross multiply: 

$c^2 - 4c = 4c+20$

Simplify into standard form: 

$c^2-8c+20 = 0$

Complete the square: 

$(c - 4)^2 + 4 = 0$

Subtract 4 from both sides: 

$(c-4)^2 = -4$

Take the square root of both sides: 

$c - 4 = \pm 2i$

Add 4 to both sides: 

$\color{brown}\boxed{ c = 4 \pm2i}$