You need to find the height, which means you have a and c. You square a and c, subtract a from c, then find the square root of that difference. Now you have b.
The height of any right triangle is just simply the many units that are going up but it has to start from the right angle and upwards and that would be your height.
It may already give you a height so there is no need to worry if so.
Here, in this case the side on the very left is my height:
