how to solve this for m?

Learn how to use parenthesis.

F = (π/2)x^2 m

You move stuff around (using the usual rules for solving equations) until m is left by itself, on one side.

F = (π/2)x^2 m
multiply both sides by 2
2F = πx^2 m
when x is not equal to zero**, you can divide both sides by x^2

2F/(x^2) = π m
divide both sides by π
(2/π)F/x^2 = m
or
2F / πx^2 = m

If it makes you feel better, you can flip left and right (the equal sign does not care which way you go)

m = 2F / πx^2

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** you cannot find a value for m, when x=0, because 0 times anything is zero

F = (1/2)πx^2 m
if x = 0, then F = 0, regardless of the value of m
0 = 0 m
m can be anything.
You cannot "solve for m" when x=0 (that is why we have to write that little caution when we divide by a variable).

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I just cheated the "lazy" way: I went to the wikipedia page about "pi" and I copied the symbol from that page, then pasted it in my answer.
π
You can also go to wikipedia pages like
https://en.wikipedia.org/wiki/Greek_alphabet
or, for mathematical symbols:
https://en.wikipedia.org/wiki/List_of_mathematical_symbols
where you copy from the first column (HTML)

Most operating systems also allow for "alt" codes, where you press the "Alt" key and - while keeping it pressed - you key in the number code for the character.
However, you have to find out what they are for YOUR system, because they do change from one system to another.

F = 1 / (2 pi x^2 m)

Multiply both sides by m
m F = 1 / ( 2 pi x^2)

divide both sides by F
m = 1 / (2F pi x^2)

https://www.wolframalpha.com/input/?i=F%3D1%2F2pix%5E2m+