This is the concept of trigonometry, to get the possible length of side a we use the sine rule which states that:
a/sin A=b/sin B=c/sin B
where;
a,b and c are the sides
A,B and C are the angles
thus the value of A could be as follows;
assuming the triangle is an isosceles triangle, the base angles will be:
(180-40)/2=70
thus;
15/sin 70=a/sin 40
a=(15sin40)/sin70
a=10.26 inches
thus the possible size of a=10.26 inches
