During an isothermal expansion, a confined ideal gas does 150 J of work against its surroundings. This implies that

From the first law of thermodynamics

                 $∆\mathrm{U} = \mathrm{Q} - \mathrm{W}$

For isothermal process, $∆\mathrm{U} = 0$

Work done against surrounding means gas expands against surrounding pressure

$\therefore \mathrm{Q} = \mathrm{W}$

Given, W = 150 J.

$\therefore \mathrm{Q} = 150 \mathrm{J}$

When  Q is positive, the heat is added to the gas.