When you have an equation and needs to solve for a variable, you need to clear that variable, as follow:
[tex]A=P+I[/tex]you need to see what is with the variable I. In this case is the P so you can clear the I by subtracting a P from both sides of the equation:
[tex]A-P=P+I-P[/tex]You make the subtraction (P-P=0) and you get:
[tex]A-P=I[/tex]Then so, the solution for I is:
[tex]I=A-P[/tex]