I need help with this problem, i literally dunno whats going on.

This time we have a bar on the x-axis lying between the positions x = a and x = b. The
density of the bar at at point x is µ(x), where µ is a continuous function on [a, b]. This means
that the density may be different at each point. Obtain a formula for the total mass of the bar
using integrals.
Note: There are two ways to look at this (which are actually the same, but some students may
understand one interpretation better than the other):
• Method 1. Start with an approximation. Take a partition of the interval [a, b]. On each
subinterval, pretend that the density of the bar is constant. Write the total mass of this
“approximated bar” as a sum. Then interpret the actual bar as a limit of this approximated
bars when the norm of the partition approaches 0. This is an integral!• Method 2. Break the bar into pieces of “infinitesimally small” width dx. Write the mass
of the bar as the “sum” (i.e. integral) of the masses of these microscopic pieces.

Can anyone help? Thanks.