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 polynomialand show that the wanted quantity m(P ) is related to a "half-Mahler" measure ofP (x, y) = P √ 7,1,3 (x, y). In the finale we use the modular parametrization of the elliptic curveP (x, y) = 0, again of conductor 21, due to Ramanujan and the Mellit-Brunault formula for the regulator of modular units.