How to calculate the enthalpy of formation of hydrazine from tabulated values?
In order to "make" hydrazine, $\ce{H2N-NH2}$ from hydrogen and nitrogen atoms, you have to


break $\ce{N2}$ once, giving 2 nitrogen atoms
break $\ce{H2}$ twice, giving 4 hydrogen atoms
form one $\ce{N-N}$ bond
form four $\ce{N-H}$ bonds


You have all the enthalpies that you need. Just pay attention to the coefficients and the signs.