whenever we are to solve the system of equations by substitution method we have to write one variable in the form of other so as to get the value of unknowns
here, we are given two expressions:
And
We have to substitute x in terms of y because the equation should be equivalent to y we will put in
we will get:
is the expression we will get after expression equivalent to y.
Because the problem gives you the value of b simply plug it in the equation a=12b. The new equation should look like this when plugged in: a=12(4.5). Now multiply 12*4.5=54. The new equation would then become a=54