Question

Уравнение: t-(7800:3900)=2000

Answer 1

t-(7800:3900)=2000;
t-2=2000;
t=2000+2;
t=2002.

А потом если хотите, делаете проверку: 2002-(7800:3900)=2002-2=2000.

Answer 2

t - (7800 ÷ 3900) =  2000
t = 2000 + (7800 + 3900)
t = 13 700