The lateral surface area of a prism is the sum of the surface areas of the sides of the prism.
Since the bases of the prism are triangles, there are three sides. The area of each lateral is the product of a side of the triangle times the height of the prism.
We can express this as Lateral Surface Area LSA = (s1xh) + (s2xh) + (s3+h), where "s1, s2, s3" are the lengths of the sides of the triangle and "h" is the height of the prism.
We can factor out "h" to get LSA = hx(s1+s2+s3) where the factor "s1+s2+s3" is the perimeter of the triangle.
Solving for "h", we get h = LSA / (s1+s2+s3)
For your specific problem, h = 300 / (4 + 5 + 6) = 300 / 15 = 20