We must know that the are of the triangle is given by definition by
A = (1/2) * (b) * (h)
where
b = base
h = height
Substituting the values
A = (1/2) * (x) * (1 + 6x)
A = (1/2) * (x + 6x ^ 2)
Since the area is 13 then
(1/2) * (x + 6x ^ 2) = 13
Clearing x we have
6x ^ 2 + x = 26
6x ^ 2 + x-26 = 0
(x-2) * (x + 13/6) = 0
As we look for the base of a triangle then x> 0
x = 2
answer
the base is x = 2
6x ^ 2 + x = 26
