how do I write an equation for the acid hydrolysis of glyceryl trimyristate (trimyristin).
using brutto formulas

Unfortunately, I've never heard of a "brutto formula", so I may not be able to help you there -- unless you teach me.

Myrstic acid is

CH_3-(CH_2)_12-RCO_2H

The trimystrin molecule is three of these carboxylic acids with ester linkages to a glycerol group. The ester links will hydrolyze like this:

R-CO_2R^' +H_2O \, \rightleftharpoons \, RCO_2H+R^'OH

to form the original carboxylic acid and an alcohol (glycerol).

So, that's how the hydrolysis works. You'll have to figure out the "brutto formula" part.