Question: coupled differential equations

how do i solve a set of coupled differential equations like these for x(t) and y(t) eqn1 := (D,2)(x)(t) = -a*sqrt(D(x)(t)^2+D(y)(t)^2)*D(x)(t), eqn2 := (D,2)(y)(t) = g-a*sqrt(D(x)(t)^2+D(y)(t)^2)*D(y)(t)
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