Multiply both Sides

eq = (x - 3 == y / 13);

lhs = eq.left * 13; 
rhs = eq.right * 13;

eq2 = (lhs == rhs);

show eq2;

Output

"eq2" → 13*(-3 + x) == y

Apply expand[ ]

eq = (x - 3 == y / 13);

lhs = eq.left * 13; 
rhs = eq.right * 13;

eq2 = (lhs == rhs);

show eq2;

lhs = expand[eq2.left]; 
rhs = eq2.right;

eq3 = (lhs == rhs);

show eq3;

save as lhsrhs;

Output

"eq2" → 13*(-3 + x) == y

"eq3" → -39 + 13*x == y