we know that
the area of triangle ABC is equal to the area of two right triangles
so
triangle ABD and triangle BDC
D is a point between point A and point C
step 1
Find the length of segment AD
Applying Pythagorean Theorem in the right triangle ABD
10^2=AD^2+8^2
100=AD^2+64
AD^2=100-64
AD^2=36
AD=6
Find teh area of triangle ABD
A=AD*BD/2
A=6*8/2
A=24 units^2
step 2
Find the area of triangle BDC
A=DC*DB/2
DC=21-6=15 units
A=15*8/2
A=60 units^2
step 3
Find teh area of triangle ABC
Adds the areas
A=24+60=84 units^2
therefore
the answer is the option A 84 units^2