This is wrong.
It's supposed to be -4/15.
But try -4/5.
Answer:

Step-by-step explanation:
First: multiply both sides by 4. 4 times c is 4c and the other sides cancels out as you are doing the inverse operation of division
Then, you have 
Subtract both sides by a squared
DO NOT TAKE THE SQUARE ROOT! This is because it is a squared plus 3b so you have to do the inverse of addition
From that you get 
Finally, divide both sides by 3.
You get 
They are both SI units of measurements