Question 61039
To solve these equations, isolate the variable to one side of the equation.  To isolate the variable, "undo" or reverse the operations:
P=a+b+c
P-a-b=a+b+c-a-b
P-a-b=c
.
E=mc^2 
(1/m)(E)=(1/m)(mc^2)
E/m=c^2
{{{sqrt(E/m)=sqrt(c^2))}}}

{{{sqrt(E/m)=c))}}}

.
y+x=a for y
y+x-x=-x+a
y=-x+a
.

3x-y=7 for y
3x-3x-y=-3x+7
-y=-3x+7
-y/-1=(-3/-1)x+(7/-1)
y=3x-7