What Concentration of HCL will have a ph of 3.5

Could you Please explain in a easy manner as my math is a bit rough.

Thanks in advance!

You have to understand log theory to do these problems. If you think that your maths is a bit rough , then swat up on logs.

First you have to calculate the [H+] at pH = 3.5

pH = -log [H+]
Using log rules:
[H+] = 10^-pH
[H+] = 10^-3.5
[H+] = 3.16*10^-4

Because HCl is a strong acid that dissociates completely,
[HCl] = [H+] , therefore
at pH 3.5 the HCl solution will have concentration 3.16*10^-4M

Let's begin with the definition of pH.
In short, pH = - log [H+], using the log base 10.
Hence we may rearrange our equation to get
log[H+] = -pH and then [H+] = 10^(-pH)

Now HCl is a very strong acid...this means it will fully dissociate in solution as follows:
HCl(aq) -> H+(aq)+ Cl- (aq)
Thus the concentration of HCl you add into the solution will be the same as that of H+ concentration in solution.

So find [H+] = 10^(-3.5) = 3.2 x 10^-4 mol/L = [HCl]