What do I do, I'm the HORRIBLE when it comes to math :'( I have a lot more of these but i need to learn how to do this.
1/a + 1/b = c for a
P= A/1+rt for r
I= kE/R for R
$\displaystyle \frac{1}{a}+\frac{1}{b}=c$
Start by removing the fractions (just multiply through by the denominators, as below):
$\displaystyle a(\frac{1}{a}+\frac{1}{b})=a\times c$
$\displaystyle 1+\frac{a}{b}=ac$
$\displaystyle b(1+\frac{a}{b})=ac\times b$
$\displaystyle b+a=abc$
Rearrange so that all terms involving 'a' are on one side of the equation:
$\displaystyle b=abc-a$
Factor out an $\displaystyle a$:
$\displaystyle b=a(bc-1)$
Divide through by the non-a factor:
$\displaystyle \frac{b}{bc-1}=a$
Try the others yourself, it's the same principle! Post your working if you're still unable to continue.