First, let's name or variables. Let us name Gracie's number of seashells as G, Nancy's number of seashells as N and Mary's number of seashells is x. The first statement would give an algebraic expression of G = x + a5. The second statement gives N = x +b1. Also, G = N hence x +a5 = x + b1. Clearly, there are lacking values noted as a and b in the equation. Given a and b we can solve for x.