Evaluate the left hand side first:
[tex] 4\cdot 45 + 3r = 180+3r [/tex]
So, the equation is
[tex] 180+3r =336[/tex]
Subtract 180 from both sides:
[tex] 3r = 336-180 = 156 [/tex]
Finally, divide both sides by 3:
[tex] r = \dfrac{156}{3} = 52 [/tex]
