5k² = 60 - 20k
5k² + 20k - 60 = 0
k² + 20/5k - 60/5 = 0 dividing by 5 at both sides
k² + 4k - 12 = 0
If we compute (k+2)², we get:
(k+2)² = k² + 2*k*2 + 2² = k² + 4k + 4
Then,
k² + 4k - 12 + 4 - 4 = 0
(k² + 4k + 4) + (-12 - 4) = 0
(k+2)² - 16 = 0
(k+2)² = 16
k + 2 =√16
This has two solutions
k + 2 = 4 or k + 2 = -4
k = 4 - 2 k = -4 - 2
k = 2 k = -6