  We prove that the (logarithmic) Mahler measure $m(P)$ of
$P(x,y)=x+1/x+y+1/y+3$ is equal to the $L$-value $2L'(E,0)$ attached to the
elliptic curve $E:P(x,y)=0$ of conductor 21. In order to do this we investigate
the measure of a more general Laurent polynomial
$P_{a,b,c}(x,y)=a(x+1/x)+b(y+1/y)+c$ and show that the wanted quantity $m(P)$
is related to a "half-Mahler" measure of $\tilde P(x,y)=P_{\sqrt{7},1,3}(x,y)$.
In the finale we use the modular parametrization of the elliptic curve $\tilde
P(x,y)=0$, again of conductor 21, due to Ramanujan and the Mellit--Brunault
formula for the regulator of modular units.
