how do i solve

Your input: 3/4-2/5*7/12

Intepretation: $\frac{3}{4}-\frac{2}{5}\cdot \frac{7}{12}$

Expand the fractions:

$=\frac{3}{4}-\frac{14}{60}$

Reduce the fractions' denominator:

$=\frac{45}{60}-\frac{14}{60}$

Subtract:

$=\frac{31}{60}$

$=0.516666...$

Q.E.D.