xp + yp = z
You are solving for p :)
So, in order to find the value of p, you must get p to it's own side, and isolate it.
Since there are two "p's," you could change "xp + yp" to "p(x + y).
p(x + y) = z
Then, we must divide both sides by x + y
p(x + y) ÷ x + y = z ÷ x + y
Simplify.
p = z/x + y
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~Hope I helped!~