? → 99mTc + β

I will write atomic number on left; mass number on right. Z is the element

xZy ==> 43Tc99 + -1e0
Left sides (subscripts) must add up on both sides. Right sides (mass number) must add up on both sides.

Therefore,
x = 43-1 = 42 and that's Mo
y = 99 + 0 = 99 so the equation is
42Mo99 =- 43Tc99 + beta