Question 289707
We have to use the area of a triangle formula, which is {{{ A=(h*b)/2 }}}
Using  the information we are given, we can substitute the numerical values.
The area: {{{ 3x^2+6x }}}
Height: {{{ 3x+6 }}}
Base: Is unknown so we can leave it as a variable b

After substituting:
The formula now becomes {{{ 3x^2+6x=(3x+6*b)/2 }}}
Multiply 2 on both sides: {{{ 6x^2+12x=3x+6*b }}}
Divide {{{ 3x+6 }}} on both sides: {{{ 2x=b }}}

Answer: The base is 2x

Check: {{{ 3x^2+6x=(3x+6*2x)/2 }}}
{{{ 3x^2+6x=(6x^2+12x)/2 }}}
{{{ 3x^2+6x=3x^2+6x }}}
Both sides are the same, therefore the answer is correct! I hope this helps! :]