The congruence ax_1x_2\cdots x_k + bx_{k+1}x_{k+2}\cdots x_{2k} \equiv c \pmod p

in a cube \mathcal B with edge length B. For a cube in general position, we show that if p \nmid abc and k \ge 5, then the congruence above has a solution in any cube with edge length B \gg p^{\frac 14 + \frac 1{2(\sqrt {k} +1.95)}+ \epsilon }. Estimates are given for the case p\vert c as well, and improvements are given for small k. For cubes cornered at the origin, 1 \le x_i \le B for all i, we obtain a solution provided only that B\gg p^{\frac 3{2k} + O\left (\frac k{\log \log p}\right )}. Under the assumption of GRH best possible estimates are given. Boxes with unequal edge lengths are also discussed.]]>