Question 1031662
The area of a triangle is given by
{{{A = (1/2)bh}}}
In our problem, b = h + 4 and A = 126...let's plug that in and solve for the height...we get
{{{126 = (1/2)(h+4)h}}}
{{{252 = h^2 + 4h}}}
{{{h^2 + 4h - 252 = 0}}}
We can factor this and get
{{{(h+18)(h-14) = 0}}}
so that 
h = 14 m and that means the base is
b = h+4 = 18 m