beta decay by emitting an electron is called as β⁻ decay. when this happens, a neutron of the element converts into a proton by emitting an electron. Hence, the mass of daughter nucleus is same as parent atom but atomic number/number of protons is higher by 1 than atomic number of parent atom.
Sr has atomic number as 38.
After β⁻ decay, the daughter nucleus will have atomic number as 38 + 1 = 39.
Hence, the daughter nucleus should be Y (Yttrium). Formula is,
₃₈⁹⁰Sr → ₃₉⁹⁰Y + ₋₁⁰β + energy