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X,Y are joint Gaussian,
∴ aX + bY is Gaussian, fo any constants a, b.
E[aX+bY] = aE[X] + bE[Y]
Var[aX+bY] = a^2 Var[X] + b^2 Var[Y] + 2abρ√(Var[X]Var[Y])

Z = -3X + 5Y
∴ E[Z] = (-3)(1)+(5)(-3) = -18
   Var[Z] = (-3)^2 (2) + 5^2 (4) + 2(-3)(5)ρ√(2×4)
          = 118 - 60√2 ρ
ρ = 0 ==> Var[Z] = 118
ρ = -0.1 ==> Var[Z] = 118 + 6√2