Yeah, I understand that too, and I guess whenever I can't think of anything, I just state the formula and use it to describe the relationships, so at least that way I get some of the marks
If you were interested though, the formula and the sin(theta) part come from the vector cross product. The length of the wire is a scalar quantity, so it doesn't matter, same as the number of wires, n, but the two vector quantities, the current, I, and the field, B, are multipled using a vector cross product (there are two ways of multiplying vectors, the cross product (which you'll do in Specialist) where you get a scalar, and the cross product (which you'll do in First Year Uni Maths), where you get a vector) - and the vector cross product is defined as the Product of the Magnitudes of the two vectors by the sin of the angle between them in the direction perpendicular to both of them.
So the Vector Cross Product explains the formula F = nIBL sin(theta) as well as the Right-Hand slap rule - finding the direction of the Force (direction perpendicular (orthagonally) to the Current and Field)
That's the maths behind it