pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 250 $mL$ $HBr$ 0.8 $M$ with $V_2$ = 400 $mL$ $HClO_4$ 0.8 $M$

$HBr$ ,$ HClO_4$ :    strong acids $n_{HBr}$ $=$ $c_{HBr}\cdot V_1$ = 0.2 $mol$ $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_2$ = 0.32 $mol$ $pH$ $=$ $-log\frac{n_{HBr} + n_{HClO_4}}{V_1+V_2}$ = 0.097