First, use the cosine theorem A 2 = B 2+C 2-2 ABCOSA to find the cosine value COSA of angle A =- 1/4, and then calculate the cosine value COSA by

(sinA)^2+(cosA)^2= 1

SinA= root number 15 divided by 4 (negative value discarded),

Then multiply the area formula s= 1/2*b*c*sinA=3 by the root number 15 and divide by 4.