The value of (4a - 3b) depends on what 'a' and 'b' are.
There are many combinations of 'a' and 'b' that make
(4a - 3b) equal to 21, and a whole bunch of others
that don't.
The equation in your question is not true OR false until you know
what 'a' and 'b' are. I suspect that right next to this problem in your
math book, it gives you some values for 'a' and 'b'.
Here are a few examples of how to do it:
1). I tell you that a=2 and b=1.
You take those numbers, and write them in place
of 'a' and 'b' in the left side of the equation:
4a - 3b
4(2) - 3(1)
8 - 3 = 5
If a=2 and b=1, then the left side of the equation is 5, not 21,
and the equation is false.
================================
2). I tell you that a=5 and b=4.
You take those numbers, and write them in place
of 'a' and 'b' in the left side of the equation:
4a - 3b
4(5) - 3(4)
20 - 12 = 8
If a=5 and b=4, then the left side of the equation is 8, not 21,
and the equation is false.
=================================
3). I tell you that a=6 and b=1.
You take those numbers, and write them in place
of 'a' and 'b' in the left side of the equation:
4a - 3b
4(6) - 3(1)
24 - 3 = 21
If a=6 and b=1, then the left side of the equation is 21,
and the equation is TRUE.
This is not the only possible pair of numbers for 'a' and 'b'
that make the equation true. There are tons of possibilities.
Look around the question in your book and see if it gives you
values for 'a' and 'b' to test in the equation.