w = m·c^2·(1/√(1 - v^2/c^2) - 1)
w / (m·c^2) = 1/√(1 - v^2/c^2) - 1
w / (m·c^2) + 1 = 1/√(1 - v^2/c^2)
(c^2·m + w)/(c^2·m) = 1/√(1 - v^2/c^2)
(c^2·m)/(c^2·m + w) = √(1 - v^2/c^2)
(c^2·m)^2 / (c^2·m + w)^2 = 1 - v^2/c^2
v^2/c^2 = 1 - (c^2·m)^2 / (c^2·m + w)^2
v^2 = c^2 - c^2·(c^2·m)^2 / (c^2·m + w)^2
v = √(c^2 - c^2·(c^2·m)^2 / (c^2·m + w)^2)