How do you ask excel to find x such that f(x,y,z)=b where y,z and b are set by you

Hi all, am new here so forgive if I'm posting in the wrong section of website !

How could you ask excel to find x such that f(x, y) = b

I mean this for a fixed b and y
A simple example of this would be if you were using the multiplication function:
f(x, y) = x*y =b
And you set inputs
Y = 5
b =40

So you want to ask excel what value x has to be to make this true (in this case the answer is x = 8 )
i.e Find x such that x*5 = 40

My question is quite general but specifically I came across this problem when using the PMT function
For those of you who don't know, PMT tells you things like what your monthly payment would have to be to payback a loan over a given amount of time periods with a specific interest rate

For example, if you borrowed €1000 from the bank at a 4.5% annual interest rate, and wished to pay this back monthly over a 20 year period.
Then in this case the monthly interest rate is 0.375%, the number of periods is 240 months, and the present value is £1000


so pmt(0.375%,240,1000) returns -6.33 which means you'd have to pay back £6.33 each month for 20 years to pay back the loan at a 4.5% annual rate

BUT what if I wanted to know what the interest rate would have to be in order to allow me to pay back £6.33 a month for 20 years on a £1000 loan

Here I'd want to ask excel:
Find x such that PMT(x,240,1000)= -6.33

which would return 0.375%

Is there a way to do this please? My question is not necessarily specific to this function but I'd just like to know how to solve problems like this.

Thank you for your help

I'm no longer on Excel computer but will try this tomorrow. Thanks for your reply have a nice day