Question 759994
The base may be contained on the lower, horizontal line of {{{y=-1}}}.  The highest point is C, so the height for the triangle is |-1|+|4|=5.


Given: Base is twice the height, so using b for base, h for height, h=5 and so {{{b=2*5=10}}}.
Base is 10 units.  


Consider the horizontal line again.  One end point for the base is B(8,-1).  We want 'a' where A is 10 units away from B.  There are two choices for this.  You can choose either {{{a=8+10}}} or {{{a=8-10}}}.  Both will work.