Answer:
[tex] 20 + (4c)c = 30 \\ 4 {c}^{2} = 30 - 20 \\ 4 {c}^{2} = 10 \\ {c}^{2} = \frac{10}{4} \\ c = \sqrt{ \frac{10}{4} } \\ c = \frac{ \sqrt{10} }{2} \\ c = \frac{5}{ \sqrt{10} } [/tex]
