Hello! So the equation you have to solve is 49 + j = 81. You want to figure out j, so you have to isolate the variable. This means, you should subtract 49. In math, if you do something to one side of the equation, you have to do it to the other. So, you must subtract 49 from 81. This means j= 32.
I hope this helps! If you need help with how to do the subtraction equation 81-49, please let me know!