Let $$A(x_1,y_1),\,B(x_2,y_2),\, C(x_3,y_3)$$ be the vertices of $$\triangle ABC$$
Let D(2,1) , F(-1,-3) and E(4,5) be the mid points of AB,AC and BC.
D and F are mid points pf ABC and AC
$$\therefore DF\parallel BE$$
E and F are mid points pf BC and Ac
$$\therefore EF\parallel BD$$
$$\therefore$$ DBEF is a parallelogram.
The diagonals of a parallelogram bisect each other i.e, both diagonals have same mid point
i.e, Midpoint BF=Midpoint of DE
$$\left( \dfrac { { x }_{ 2 }+(-1) }{ 2 } ,\dfrac { { y }_{ 2 }+(-3) }{ 2 } \right) =\left( \dfrac { 2+4 }{ 2 } ,\dfrac { 1+5 }{ 2 } \right) \\ \therefore \dfrac { { x }_{ 2 }+(-1) }{ 2 } =\dfrac { 2+4 }{ 2 } \quad \therefore { x }_{ 2 }=7$$
Similarly $$\dfrac { { y }_{ 2 }+(-3) }{ 2 } =\dfrac { 1+5 }{ 2 } \quad \therefore { y }_{ 2 }=9\\ i.e,\left( { x }_{ 2 },{ y }_{ 2 } \right) =\left( 7,9 \right) $$
D is the mid point of AB
$$D=(2,1)=\left( \dfrac { { x }_{ 1 }+{ x }_{ 2 } }{ 2 } ,\dfrac { { y }_{ 1 }+{ y }_{ 2 } }{ 2 } \right) \\ \dfrac { { x }_{ 1 }+7 }{ 2 } =2\quad \quad \quad \therefore { \quad x }_{ 1 }=-3\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad i.e,({ x }_{ 1 },{ y }_{ 1 })=\left( -3,-7 \right) \\ \dfrac { { y }_{ 1 }+9 }{ 2 } =1\quad \therefore { y }_{ 1 }=-7$$
F is the midpoint of AC
$$F=\left( -1,-3 \right) =\left( \dfrac { { x }_{ 1 }+{ x }_{ 3 } }{ 2 } ,\dfrac { { y }_{ 1 }+{ y }_{ 3 } }{ 2 } \right) \\ -1=\dfrac { -3+{ x }_{ 3 } }{ 2 } \quad \therefore \quad { x }_{ 3 }=1\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad i.e,({ x }_{ 3 },{ y }_{ 3 })=(1,1)\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \\ -3=\dfrac { -7+{ y }_{ 3 } }{ 2 } \quad \therefore \quad { y }_{ 3 }=1$$
$$\therefore$$ The vertices of triangle are $$=(-3,-7),(7,9),(1,1)$$