Question 1082846
<pre>
The standard quadratic formula in vertex form is

{{{matrix( 1,13,     "f(x)","",""="","",a*(x-h),""+"",k,",",where,"(h,k)",is,the,"vertex.")}}}

h=-3, k=-6

{{{matrix( 1,13,     "f(x)","",""="","",a*(x-(-3)^""),""+"",(-6),",",where,"(-3,-6)",is,the,"vertex.")}}}

{{{matrix( 1,7,     "f(x)","",""="","",a*(x+3),""-"",6)}}}

Since (0,–15) is a point on the graph, then f(0) = -15

{{{matrix( 1,7,     "f(0)","",""="","",a*(0+3),""-"",6)}}}

{{{matrix( 1,7,     -15,"",""="","",a*(3),""-"",6)}}}

{{{matrix( 1,7,     -15,"",""="","",3a,""-"",6)}}}

{{{matrix( 1,5,     -21,"",""="","",3a)}}}

{{{matrix( 1,5,     -7,"",""="","",a)}}}

So the equation 

{{{matrix( 1,7,     "f(x)","",""="","",a*(x+3),""-"",6)}}}

becomes

{{{matrix( 1,7,     "f(x)","",""="","",-7*(x+3),""-"",6)}}}

Edwin</pre>