I think you are referring to the following paper:
Gerstenhaber, Murray; Rothaus, Oscar S.
The solution of sets of equations in groups.
Proc. Nat. Acad. Sci. U.S.A. 48 1962 1531–1533.
Also of interest (and more easily available, for instance on the arXiv) is the following:
Klyachko, A. A.
The Kervaire-Laudenbach conjecture and presentations of simple groups. (Russian. Russian summary)
Algebra Logika 44 (2005), no. 4, 399--437, 512; translation in
Algebra Logic 44 (2005), no. 4, 219–242
The term you want to search for is the "Kervaire-Laudenbach conjecture".
Edit: I found these slides by Andreas Thom, which include a sketch of the proof of the Gerstenhaber-Rothaus theorem and an indication as to how solving equations over groups is related to the possibility of making a non-trivial group a trivial one by adding one generator and one relation.