It depends where the right angle is. If the right angle is at O then you can situate the triangle this way
with a base length of 5 units and a height of 3 units.
The area is then 1/2 base height .
If the right angle is at B then you can draw the triangle this way
and use Pythagoras theorem to find the length of the base BA before applying the area formula.
Can the right angle be at A? If so what do you do?