Turn it into a "two-equation, two-unknown" setup, and knock it out that way.
With m playing the role of the market expected return, and r starring as the risk-free rate, Wal-Mart's expected return sets up as...
0.12 = r + 1.1(m - r)
...and Dell's equation is
0.123 = r + 1.2(m - r)
Solve that system for m and for r, and you'll see it agrees with the given answers. Cheers!