How to solve this Calculus question

I need help with this. Can't seem to get the answer.

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THANK YOU

Given that : st+t^2 = 4, find, d^2t/ds^2 = ?

t + sdt/ds + 2t.dt/ds = 0,
(dt/ds)[s+2t] = -t,

dt/ds = -t/(s+2t) ........................................… [1],

d2t/ds2 = -(dt/ds)/(s+2t) + [t/(s+2t)^2]*(1+2.dt/ds),

From [1],
d2t/ds2 = t/(s+2t)^2 +[t/(s+2t)^2]*(1-2t/(s+2t)],

d2t/ds2 = [t/(s+2t)^2]*[1 +1 -2t/(s+2t)]

d2t/ds2 = [t/(s+2t)^2]*[2s+4t -2t]/(s+2t),

d2t/ds2 = 2t(s+t) / (s+2t)^3 >===================< ANSWER