right angled triangle used height is h and length is h+6

A_{triangle} = (b*h)/2

If we plug h=h and b=h+6 into the above formula, we get:

A_{triangle} = ((h+6)*h)/2

= (h^2+6h)/2 distribute h across (h+6)

If we plug h=12 into the above formula, we get:

A_{triangle} = ((12)^2+6(12))/2

= (144+6(12))/2 squared 12

= (144+72)/2 multiplied 6*12

= 216/2 added 144+72

= 108 divided 216 by 2